The prelude and fugue gets discussed after the break.
Prelude (starts at 0:20)
The prelude here is an aggressive, relentless perpetual piece with running sixteenth note activity throughout, that starts in the bass line. A somber quarter-tone melody floats above the perpetual motion. Perhaps the simplest way to describe this menacing but charming prelude is as a march in a strophic/binary form. The marching chorale tune appears in the treble strings in the first half and in the second half in the bass strings while the perpetual motion switches from being in the bass strings to the treble strings. Being in the relative minor to the preceding prelude and fugue in C major this companion piece in the cycle is ominous and agitated where the C major dyad was calm and serene. Each half of this prelude has a tonic and dominant phrase that gets rounded off with a cadential push into the next phrase. In this case the prelude ends with an attaca, incomplete cadential gesture that leads with a very brief pause into ...
Fugue (starts at 1:54)
This fugue opens with an ominous and languid subject that moves by fifths across the root, fifth and ninth. For those who don't have the scores at hand this subject seems like one where some kind of written description will help elucidate what you can hear at the start.
A E B F (pause) B C D | C G D A (pause) D C B | A E B F (pause) B C D | C G D A (pause)
fourth measure closes with what can be thought of as a locrian flourish in sixteenth notes.
This is another fugal subject with a fairly wide ambit, an eleventh rather than a twelfth. While in more parochial terms this could be considered a difficult fugal subject what makes the large ambit work is that although the subject has a large range it is spacious, particularly at the tempo Koshkin calls for. There's a lot of room for rhythmic variety. This will become audibly clear as the voices enter in the exposition.
This is, like many of the fugues in the cycle, a three-voiced fugue. There are, however, four statements of the subject. One of the many things you can do with a fugal exposition is introduce additional entries of a subject before completing an exposition if you have a subject that lends itself to an extra statement.
So we have the following:
1. Subject appears in tenor
2. Subject is answered at dominant in alto
3. Subject appears in soprano
4. Subject appears in tenor/bass at the dominant.
In this case the parallelism of the four measure phrases builds a momentum that I think needs to be respected, and Koshkin respects it. The reason for this has to do with the harmonic movement in the subject. Despite the quintal activity in the subject the root movement progression is between the minor tonic and the major mediant. Given the ambiguity possible in a subject so anchored to mediant root movement, offsetting that potential for ambiguity by reinforcing the tonic dominant relationship in the entries of the fugal subject within an extended exposition is a very wise compositional move. It also lays down what I'd call a kind of rock and roll momentum for the subject at hand, because if anyone's hear The Police song "Message in a Bottle" this has a comparable kind of vibe in intervallic terms, though Koshkin's subject is more menacing and substantially less repetitive.
This fugue also introduces what I would call a true countersubject, the descending minor third in sixteenths that leads to a briefly repeated tonic or dominant pedal tone depending on whether we're in a statement of the subject or its answer at the dominant. That this countersubject is dropped in the fourth entry of the subject, the second entry of the subject at the dominant, is a signal that the exposition is wrapping up and the development of the fugue is beginning, which is also a fine compositional move.
After the episodic work begins at about 2:30 we get to a very fun passage at about 3:12 where Koshkin introduces material that is derived from his subject and countersubject that is in the whole tone scale.
At 3:33 we get the first middle entry, in E minor. This is followed immediately by another middle entry in the tonic key. Now among those who teach counterpoint for any instrument but the guitar there might be a complaint here that if you observe the old Baroque ideal sometimes called a "tour of keys" model for fugal composition nobody should be getting back to the tonic key so quickly. Sure, there's that theoretical and philosophical complaint that could be made, but seeing as so few guitarist composers the world over have composed cycles of fugues I honestly don't see this as something to be pedantic about unless you're willing to compose a fugue in A minor with a middle entry in B flat major.
So there are just a couple of middle entries in the dominant and tonic keys before a structural climax is achieved in the 90s measures. The subject and countersubject re-appear in climactic form at 4:19. This is followed up by a number of flourishes on the locrian riff from the end of the subject. At 4:51 (measure 102) we hear the subject enter at the dominant in the lowest voice and get answered at the tonic by the upper voice. By this point the fugue, though formally having three voices, has had a two-voiced texture in many areas. As George Oldroyd put it in The Technique and Spirit of Fugue one of the mistakes contrapuntists make at the student level is wrongly assuming that once you introduce X number of voices that X number of voices must always be active throughout the duration of the fugue. Koshkin does not make that mistake here. Given the liveliness of his countersubject it makes sense to let the spacious subject be offset by an active countersubject and let two voices convey a contrapuntal richness that the guitar can certainly imply but cannot necessarily execute in a flesh and blood performance.
Now at 5:04 Koshkin introduces another stretto point. He's not composing grand stretto passages but what he does introduce here is the subject in the upper voice which is answered in imitation by the inversion of the subject in the lower voice. After this closing stretto passage Koshkin presents the subject in augmentation with percussion effects. Then the augmented quintal subject is presented again with its tonic to mediant harmonic movement as a kind of grand chorale (though one that relies on the natural resonance of the instrument to convey its effect rather than attempting to present all of these stacked fifths as a single harmonic/melodic moment. There's a brief semi-comic semi-menacing pizzicato passage, and then Koshkin closes the fugue on a gentle A major chord, bringing a fugue full of agitation and gloom to a paradoxically sunny, gentle conclusion.