Reporting back, hopefully for the benefit of others.
I've now had the Union 6 Supremes for about a year, with a Decware Zen Triode (Level 1) and Schiit Kara (1st gen) preamp. I listen in a small room, never go above 80 dB, mostly in the 60s dB range.
When I got the Zus, I thought they were too "lean". The deep bass was there and was fast (more on this shortly), but something in the low mids was missing. There was just no heft to the music. My previous pair of speakers were Zu Dirty Weekend II, and those had a very thick sound that I got used to. It is quite possible that someone else would think that the Union/Zen combo is properly 'audiophile', but that just wasn't my taste. Also, there was something about the Union/Zen combo that just didn't sound quite real to me. I believe it was the same issue, I expected certain instruments and voices to have more heft to them "in the real world." This was particularly the case with male voices and string instruments, which sounded too reedy to me, in many contexts. (Like everything, this can also be nice sometimes: for example, I think what's amazing about Thom Yorke's voice was actually highlighted by a slightly reedy presentation. But the richness of Sam Beam's voice (Iron & Wine) was destroyed.)
That said, the detail retrieval with Union/Zen is amazing! Highs are smooth and precise, LOTS of air and instrument separation. For example, you could easily hear nuances in echo and room information like the distance of Black Francis's (Pixies) voice from the mic on "Hey". Really great stuff with lots of a-ha moments. But there was no 'meat' to the music. I tried tube rolling with advice from Brent Jesse, and it made some difference vis-a-vis the heft. But not enough.
As time went on, I got more and more used to it, or perhaps the speakers broke in more and more. But the overall leanness never went away. This might just be my preference, of course. Others might find this combo exactly right. It is definitely revealing and quite pleasing, just not exactly what I wanted.
I assumed that the Union 6 Supremes were the main culprit. But since speakers are hard to change, and I foolishly got rid of all my other speakers in order to afford the Unions, I was constrained to experiment with amp changes. So, as of two weeks ago, I've been using a First Watt SIT-3 instead of the Decware Zen. This was an un-auditioned purchase, just given what people have said about that amp online.
OMG, night and day difference. The music has weight! It is exactly what I was missing. Everything just sounded thicker. (But keep reading.) The center of gravity of the spectrum with the SIT-3 is definitely towards the low-end, where the center of gravity with the Zen is towards the top-end. The music sounds 'anchored' with the SIT-3 in a way that it wasn't with the Zen. Of course, these are all just metaphors for something being heavy, but that's what the SIT-3 sounds like to me. Also, I've used a ZROCK2 in the Zen chain. It adds to the deep bass, but does not add .
This weight means that I can play the SIT-3 at insanely low levels, and it still sounds good -- tonally the same as loud volumes (I mean like mid 40s dB; middle of the night, in the dark listening). That aspect of the Union/SIT-3 combo is kinda astounding. I may keep the SIT-3 just for this, as I often listen to music when my wife has gone to sleep.
But nothing is free! Detail retrieval is *nowhere near* that of the Union/Zen pairing. I hate to say that Union/SIT-3 is murky in comparison to the Union/Zen, because that's the sort of thing that can be blown out of proportion. But it is in that direction. It is still very pleasant, and I do find myself wanting to listen more and more, but subtle information like echo or the attack of a piano key/drums are just less there. But, to be clear: still very pleasant, and I may ultimately prefer the Union/SIT-3 combo. Still thinking about it.
What is really surprising about these comparisons is that the deep bass is faster with the Union/Zen than it is with Union/SIT-3. With the former, I could feel kick drums (and, actually, drums in general) in a way that the latter sublimated. The separation of bass from the rest of the music is also clearer in Union/Zen. The Union/SIT-3 bass is robust and thick, with nice tone. But it isn't detailed. The Zen really is an amazing detail machine.
So, the adage that the Unions reveal what's in the upstream chain is insanely true. I got the SIT-3 in order to add weight to the music, but, as of this moment, I think it adds a bit too much weight. It might still win for me in the long run, but it would not be a win without casualties.
I haven't said anything about holography or sound-stage yet. My first impression is that neither the Zen nor SIT-3 is significantly better, just different. (With the caveat that instrument separation on Zen is clearer.) Both have nice expansive stages. I guess SIT-3 is a little wider and feels more immersive (again, I think the 'heft' of the music is what does the work here.) But I wouldn't complain about either of them.
I'm very curious whether other First Watt amps would be the right combo with the Unions, or even something like Pass XA25. And I keep oscillating between what I prefer: when I first put SIT-3 in the chain, I thought it was way too murky. A few days later, I loved it and thought that my end-game amp had arrived. Today, I'm going back and forth.... Break-in is not an issue with almost a decade old SIT-3, so the sound is not going to change, but my opinions keep changing.
Finally, I haven't mentioned anything about the Kara yet. I thought it was pretty invisible in the Union/Zen chain. But I wondered if the SIT-3 can benefit from a different pre-amp, as some folk online have suggested. So I tried a Linear Tube Audio MZ3 instead of the Kara. I haven't had a chance to A/B the Kara and MZ3. But neither preamp changed the overall presentation of the amps from what I wrote above.
Anyway, hope this is useful to someone. I really don't know if I'll keep the SIT-3. I wish I could compare it directly to more amps...