I listened to it again, with the 6 voice fugue afterwards, and it's a really beautiful diptych. My impressions are very... impressionistic and un-technical.
First of all, I dig the two pieces together so much more than individually (and I like them individually as well!). The fragmented character of the lamentoso comes to unity in the fugue, providing balance and, in a sense, closure. I vaguely remembered the pentatonic theme/atmosphere from the first movement, and I was happy to hear it resurface in the perhaps most beautiful episode of the lamentoso at 170-196, but I forgot that it becomes the first theme of the fugue! and I forgot that towards the end the fugue also transitions into pentatonic territory, so seamlessly and beautifully. Wonderful. Given that I seem to remember that the first movement has a lot of pentatonic elements, I feel that this gives such a strong narrative force to the sextett and I can't wait to have the time to listen to first and second movement consecutively in one session. Also, forgot that the Razumowsky-esque "sighs" reappear in the fugue as well.
Other observations: it's remarkable how you keep very tightly together a movement that not only has many themes but also many "souls". For instance, I like very much that (it might be my own feeling, unintended by the composer, of course) at the beginning (around 34ff) and then later on (147-155), the music and the singing of the voices become somewhat operatic. I really like how the triplets at the beginning are unexpected and really create sudden tension, drama, and darken a mood already dark. I like the high cello register at 47 and the dialogue with the other voices that follows it. However at 61-67 maybe it would be possible to have less intensity, fewer voices conversing, and then, as it is, have them all join together in the 74ff episode. I really like the powerfully expressive, late-beethovenian trills at around 110 (also appearing in a similar fashion in the fugue, though more regimented there). So to me there is the general sturm-und-drang tension and drama, but also opera-like singing and recitativi, there's the dialogue between classical harmony and pentatonic episodes (again so beautifully and seamlessly driving one into the other, esp in the fugue), and other things I'm surely forgetting now, or I didn't even realize they're there... and although all this wealth sometimes does feel a bit like a bunch of siblings fighting for attention (but they're all so cool and beautiful and well behaved), in the end they all coalesce and are elevated in the magisterial 6 part counterpoint. Can't wait to listen to the next chapter of this beautiful journey!