Now for this one there is not yet any video of a performance of either the prelude or the fugue so we have to discuss this one relying on the score alone. The specified tempo is adagio (quarter equals 64 bpm). This pair of pieces calls for drop D tuning and it doesn't take long to discover why.
The prelude is a pastoral in 3/4. I will propose that what we have here can be thought of as being a ternary form with 3/4 sections at measures 1-16 and 25-39 in the outer sections with a middle section of measures 17-24 that is scored in 3/4 but is tacitly in 2/4 and that should be interpreted as being in 2/4.
The opening gesture is a quartet of sixteenth notes with a jumping pattern between B naturals. The first is open string, the second and third are in the octave above, and the fourth is open string again. This opening quarter note is on the tonic and the subsequent half note chord is a minor seventh supertoinc with an omitted fifth. If we can see past the sixteenth note flourishes that nevertheless define this prelude we can see and hear that this prelude is a kind of chorale. Take the downbeat of each measure and the first note in the upper voice is paired in a tenth relationship with the bass line throughout the opening phrase. Despite some dissonant notes throughout the phrase of the sort we can expect from Koshkin, and a modal mutation into parallel minor bubbling up throughout even this opening phrase, this overarching chorale in tenths is easy to perceive in the score and even easier to perceive when you play through this.
The second phrase starts on the relative minor and gently arcs through E minor seventh, D major, G minor seventh, a trio of notes in descending order of C, D and A, then G minor in root position followed by C minor and B flat (which can be construed as G minor in second inversion but because of the G natural that appears in the treble, but here in measure 7 it could as easily have been an F rather than a G) before preparing a rolled chord half cadence at measure 8. We have an initial phrase and a continuation phrase for this pastoral chorale. The tenths remain the guiding musical element in this second phrase as it will throughout. In this second phrase the sixteenth notes shift from low notes rising to higher notes and falling to higher notes dropping to lower notes before returning to the higher origin point in the phrase. I.e. the motion is inverted and the direction of the melodic movement gets flipped for phrase two. It will stay "flipped" until we get to measure 17.
By the third phrase we get a quasi-jazz presentation of something that shows up a lot in Baroque music, a chain of circle progressions. G minor rises to C minor, then falls to F minor seventh which rises to a second inversion G minor triad which underscores that it "could" have been a B flat major triad but this prelude is in G major, not B flat major. Then we get E flat dominant seventh leading to an A NATURAL minor seventh chord with an omitted fifth. As chains of circle progressions go this is classic stuff, some who abjure tonal music would see it's a Baroque cliché. Well, sure, it "can" be but I like the steady use of minor seventh chords in this chain of circle progressions and relative minor feints. Koshkin gets this whole sequence of phrases to work elegantly and beautifully. He wraps up this sequence in measure twelve by preparing a G dominant seventh chord that doesn't continue the chain of circle progressions into C major or C minor but instead modally mutates into a G minor signaling the end of this phrase.
From measure 13 on he transforms the sixteenth note flourish into a developmental phrase where the previous chains of circle progressions are substituted for sixth root shifts while the melodic activity gets more animated. Now at a practical level I would recommend there are two different ways to approach this passage. The first way is to assume the half notes in the bass line are impossible to play as written, in which case you treat each half note in each phrase from measures 13-15 as functionally indicating a quarter note rather than a half note and cheating by leaning on the decay of the guitar's sound. If you take this approach the passage is really quite easy to play, it just doesn't convey the WRITTEN note values in the bass line.
But if you want to observe the half notes as written this is where the scordatura comes in. Starting at measure 13 play a full barre at the tenth fret and place your fourth and third fingers (or whichever you'll find easier for this) on the thirteenth frets of the fourth and fifth strings. Execute the four sixteenth notes at the start of measure 13 and then shift the fingers you use on strings four and five down to the A natural while maintaining the barre. Thanks to the full barre you're already positioned to play the D natural. The G natural can be taken by the fourth finger. Personally I would recommend you take the sixteenth notes in mm 13-15 staccato for clearer articulation of that idea, which lets you use barre (full or partial) to maintain the continuity of the bass line.
Now when we get to measure fourteen I recommend a barre at the seventh fret so that the D and A can be taken on strings three and four while the second finger of the left hand takes the B flat on the sixth string. With that modification in mind you can replicate the left hand formation from before, in which the quarter note on beat 2 (A in m 13 and G in m 14) is taken by the third finger of the left hand while the minor seventh above (i.e. the sixteenth notes in the middle of the beat) are taken by the fourth finger of the left hand. The C flat (i.e. B natural) is already part of the barre you've formed and the B flat can be taken by the fourth finger releasing the F. As long as you keep in mind the scordatura allows for it, the barre at each measure makes this passage much easier to play than the sheer impossibility you get if this were in standard tuning.
I'm not going to say this passage is EASY but an experienced and knowledgeable guitarist will know how to take up this passage and observe that it is playable with the drop D tuning in mind. At the risk of pointing out the obvious, the Adagio tempo for this one is your friend! For measures 13 through 16 the bass line, not the treble strings, is where the real melodic activity is. Play it that way. At the risk of just saying I've played guitar for twenty-five years and know a few things about contrapuntal literature for guitar, if you have to take a shortcut in this passage go staccato on the sixteenth notes and use barre formations that let you emphasize the melodic activity of the bass line here.
Now, about measure fifteen, the left hand movement works out best if you replicate what I described for measure 14. If you take this overall approach then you're using barre chords and your second finger planted on the sixth string to sequence down the fingerboard in a way that makes the whole passage a stretching exercise but one that makes the passage eminently playable, even if you might want to use staccato on those sixteenth notes for articulation. If you try to play them at full value and forget that the bass line is the primary melody this passage will be obscured. If you want to really accent the melodic movement in the bass line from measures 14 to 15 you can use finger 2 of the left hand and portamento down to the A flat from the C natural at the end of the measure. It's not written in the score, obviously, but those of us familiar with Koshkin's work know that there's not just Shostakovich and Stravinsky influencing his music, he's also got some familiarity with Led Zeppelin and popular styles. To play Koshkin's music effectively it helps to be aware of not just classical guitar techniques but a few rock guitar techniques, too. So ...
This challenging but still idiomatic passage culminates in a cadential turn at measure 16 that could be hard to play if you don't have a very generous handspan with flexible fingers. I find it's easy to play by anchoring "1" on the C sharp and stretching my "2" finger up to the B flat on the fifth string and continuing the phrase throughout the measure but I can play elevenths with one hand for both my hands, which not many people can do. Given how rapid the decay of tone on the guitar is, however, if a guitarist wanted to cheat a bit you can release the C sharp on string 2 at the second half of beat 2 and reposition your left hand to complete the necessary melodic turn in the bass at measure 16. That's probably the best practical advice for this passage.
That passage is a half cadence preparation for an episode that starts at measure 17 in D minor and goes through measure 24. Koshkin reintroduces the original form of the "jump base" melodic figure in sixteenth notes in a quasi-stretto episode.
A word about the sixteenth note runs in measures 17 through 24: treat the LOWER notes as if they are really indicating a dotted eighth note and a sixteenth note figuration. Play the UPPER notes staccato to highlight this. At measure 19 it should be obvious C is taken on string six so that the E flats can be taken on the third and first strings. This lets you shift effortlessly from the lower E flat on the third string to the F above while taking the D and B natural on the open strings around it. Baby simple there as long as you understand how the scordatura has set up this passage.
I would advise playing measures 17 through 24 as if they were in 2/4 not 3/4. It highlights how this entire passage is a middle episode in the prelude and the implied dotted eighth and sixteenth note pattern in the middle voice unites with the bass to form that melodic/harmonic tenths pattern that is the hallmark of this prelude. In all of the initial 3/4 there's been something about this prelude that is written in 3 but that "wants" to be in 2 and I propose mm 17-24 is where the duple pattern lurking within this prelude gets to briefly emerge. This is something I recommend, additionally, because when we get to the fugue and its subject it's going to turn out this is the four-on-the-floor duple pattern that permeates the fugue. The prelude is supposed to be a prelude, after all, and interpreting the elements of the prelude which best foreshadow the fugue can be kept in mind when interpreting a prelude that is not self-contained but anticipates a fugue.
Which is why, by the way, I recommend that you play the last beat of measure 20 by taking that A natural on the fourth string and playing the D that "could" be played open on the fourth at the fifth string. This lets you sustain the implied dotted eighth and sixteenth pattern for the A natural on the fourth string and maintain the tenth between the A and the C above it. You can then easily shift to measure 21 with little fuss. Again, take all of measures 17 through 24 as if they're really in 2/4, the 3/4 meter withstanding, and this passage can really sing. Lean into the recommended ritardando in measure 24. I would even go so far as to advise a fermata on the last beat of 24. This is the half cadence that prepares for the recapitulation of the initial pastoral waltz.
Measure 25. If you needed any evidence that the real melodic content is in the tenths the chromatic embellishments in the bass line really spell this out for us here. Using 18th century music as a precedent, you don't embellish the second time around what hasn't been thematic/melodic material, as a rather general rule. I would recommend treating measure 27 and the first half of measure 28 as a measure of 4/4 and the half note in the rest of measure 27 as having a fermata. Although we've returned to 3/4 let’s say that the march that is hidden in this prelude still wants to get out and this can be thought of as a "last hurrah" push from duple meter in this prelude before it returns to 3/4.
Measure 29 is where I think we're firmly back in waltz mode. The opening chord is easy if you take the C sharp on the sixth string, play the obviously open notes on their associated strings and take the B flat on the second string. Take the half note chord with your third finger planted on the fifth string for the lowermost D natural, which gives you G on the open string and lets you prepare in advance with the F sharp on the fourth string. Measure 30 can be taken entirely in first provision with the caveat that you may want to let the E natural go after beat 2 is done so that you can take up C natural on the second string for beat three. There are, no doubt, other ways to take the passage but this is the solution that will demand less stretching and strength from the left hand in my experience. It also lets you sustain the G natural at the top for its full duration. Everything else seems pretty straightforward in this coda except for measure 34. Given the decay rate of the guitar I think you can release the chord in the trebles and take the third beat of the measure as a sixth string solo.
Now obviously you can write a few thousand words about what amounts to just barely more than one page of music, even a piece that may be just under two minutes long. One of the things I find appealing about Koshkin's music is how he evokes a mercurial balance of whimsy and menace in his work. This prelude in G major manages to be a good case study. The sixteenth note gesture that leaps back and forth across octaves will prove to be a gestural anticipation of the fugue and while it isn't what I'd regard as the core melodic interest for the prelude itself it's important to accent that material all the same because it is the gesture that most emphatically foreshadows the fugue that follows.
What's worth emphasizing as a pattern throughout the cycle is that when the prelude ends there's no "attaca", there's not even a final bar line measure indication. The scoring simply continues measure numbering into the fugue as something that is inextricably tied to and following the prelude. So while there is a cadence on the tonic at the end of the prelude it leads with just one measure's pause into the fugue.
The fugue is labeled "Allegretto" and has a spunky marching subject in common time. It can be thought of as a "ramp" subject. There are "wedge" subjects that have a starting or ending point to which or from which the melody expands or contracts by either diatonic or, as often, chromatically expanding intervals. I call this fugue subject a "ramp" subject because the leaps start as a G to G octave leap that contracts into smaller intervals with each leap. G to G, G to F sharp, G to F natural, G to E natural, G to E flat, G to C sharp, G to D breaks the cycle but then we're back to G to C natural and then G to B flat rounding off a three-measure subject that has a ragtime like jump-bass intervallic pattern.
The subject is given an answer at the dominant and it has a real countersubject which emphasizes the rhythmic pattern of an eighth note followed by a quarter note followed by an eighth note or an embellishment of said rhythmic duration. While "allegretto" might seem a bit fast this lively march-like fugue can be thought of as a quasi-ragtime fugue. It's cast in three voices and has a countersubject that appears at measures 43-45.
A brief word about the entrance of the second voice, Koshkin simply jumps in with the entrance of the second voice as soon as he has completed the three-measure subject. In this case I would say that the open fifth followed by a leap via parallelism to another open fifth could have been avoided by way of a rising one measure melodic transition. There's no obligation for each entry of voices in a fugue to be direct when you can avoid parallel motion by introducing interrupting transitions. A fourth measure with a rising transition passage preceding the entrance of the second voice with the answer would be fun here and it could introduce a nice musical "gotcha" moment where listeners might be fooled into thinking the subject is four measures long only to discover that the subject is just three measures long. Everything else about the fugue exposition could remain as written.
The development of the fugue commences at measure 49 and leads to a middle entry in D major at measure 53. Here we'll see that Koshkin doesn't include the augmented fourth featured at the end of the second measure of his subject for the obvious reason that between the fifth and sixth strings being tuned to A and D respectively there's no place to have a G sharp or A flat. So he substitutes a repetition of B flat where G sharp or A flat would have been in the second half of measure 54. When writing fugues at a practical level you will find, especially on a guitar, that a subject in a middle entry may have to be modified at a few places in order to produce actually playable music.
The next episode begins at measure 57 and feints at a return to the key of G major but quickly modulates to a new middle entry in C major. This leads by way of a mixed mode Lydian episode into a new middle entry in F major that runs from measure 65 to 67. After a half measure twist we get the second half of measure 68 introducing a new episodic middle entry in C in which the subject appears in C but is answered half a measure later at the start of measure 69 with a new entry of the subject a tenth higher at E flat. This can be thought of as a polytonal canonic middle entry. It's featuring a subject at C that has a few omitted notes but at a practical level a listener and a performer can "hear" those notes even where they are not possible to practically be played. Koshkin has set up enough middle entries at this point that some elements of a contrapuntal texture can be implied where they cannot be played. This happens even in a number of Bach's fugues for solo violin so it's no surprise if that's what we can infer is going on in this middle entry in one of Koshkin's fugues.
Thanks to the aggressively chromatic nature of the subject modulating episodes can be effected in short order. A mere two measures allow Koshkin to modulate into his next middle entry, in E major. This middle entry is immediately followed by another middle entry in B minor (measures 76-78). From there Koshkin moves along to a climax at measure 83 in which he sequences the initial gesture of his subject above a pulsing dominant pedal point on the sixth string as the fragment of the subject is sequenced through a chain of thirds from A flat to F to D to B before bringing back the countersubject as another embellishment of a half cadence prolongation that finally resolves to fragments of the subject in G major at measure 93, which could be considered the syntactic/tonal resolution of the fugue ...
but the subject appears in fragmentary form. It's bandied about in a tonic and subdominant alternating pattern for a bit and when we finally get a full presentation of the subject it's at measures 97-99 where it is presented in inversion. Even at measure 99, however, the subject is still cut off to make way for the marching coda that rounds off the fugue, evoking a duple meter form of the coda materials that concluded (somewhat) the initial prelude). That chord in harmonics at the penultimate measure can be played as a barre across either the fourth or ninth fret depending on what's easier for you. I would recommend a short fermata there to accentuate the not-in-the-key nature of the chord on the one hand and, on the other, to give a performer who isn't already used to playing chords that consist only of natural harmonics time to let the chord ring before the closing chords.