I just lent out the Vali to ultrabike since he has a HD558 and we were curious how that combo would fair.
First off, let me say: the Vali is an incredible amp with the right headphones, and I agree almost 100% with your assessment of the vibrancy, immediancy, excitement, and general sound signature.
In regards to 558 + Vali: Very badly, in my experience. I used to own the Vali and paired it with the 558 back in the day. Soundstage collapsed, drums got pushed back way in the mix and became extremely wimpy sounding, the entire sound signature became a bit tinny, and the clarity dropped and became very muffled.
Sold the Vali eventually to fund a Valhalla 2 (to a guy who needed to get into higher end audio) and loved the Valhalla 2. Last week I found a Vali again for sale for cheap and jumped at the opportunity to own it again. I plugged in my 558 after reading your assessment of the pairing and, yep, my original experience with the pairing came right back.
Between the 600 and 800 on the Vali: I think the 600 pairs better, but I'm still experimenting so the jury is out on that.
Between the Vali and the Valhalla 2.....I think I like the Vali more, honestly. It has a magic that the Valhalla 2 is lacking. The energy, the sense of rhythm, the way it handles vocals, the depth of the soundstage....can't be touched by the Valhalla 2 (w/ stock tubes). They're completely different listening experiences. The Vali sounds more "real" for a lack of better word, more visceral. I get the sense that the soundstage is filled by living, breathing things. There's a lot of weight and body to the instruments. Vocals just float and hang in the air in front of you. It's incredible. The Valhalla 2 is more detailed, with more air, space and wider soundstage (cleaner and clearer sounding) but sounds like you're listening to a recording.
Anyway, I'm glad to be back on the Vali train.