Well, Bob from Eurotubes agrees with Woodman, Aunchaki et. al.:
"You will likely notice more character from preamp tubes at lower volumes, while more moderate to higher volume settings will open up the power tubes"
Also, he suggested a full retube (which is probably what I'll do anyway). Are you as shocked as me?
Honestly, he's probably right, and no self respecting bidnessman would have done otherwise.