The first difference was vocals - for example in rocks off or tumbling dice on Exile on Main Street, they'd been recessed a bit, almost lost in the mix, with the electro harmonix. I'd say a certain natural prominence I'd always heard was gone. Also in From Her to Eternity by Nick Cave. When I put the Psvane into the V1 slot, I didn't hear a difference. When I put them into the V2, it was an immediate improvement. I'd also say the soundstage expanded, and some textures on the vocals were there that weren't there before.
Additionally, the sound was not as bright, which in my system with Thiel 1.6, brightness can be an issue.
btw, I also went from the 8 ohm to 4 ohm taps, and that's really helped on the coherence. The Dvorak dumky trio on a recording by Trio Wanderer has more image solidity and naturalness using the 4 ohm taps.
Thanks for your kind words on the VAC. It's a fantastic amp, and I'm just tuning it right now. I feel I can get more out of it.