I don't know the answer to your specific questions, but I do know that Dave Friedman has mentioned multiple times on Tonetalk that on more than one occasion he's been able to make two amps (of the same model) with different power tubes sound identical with different knob settings. I think a lot of guitarists make an amp's power tube type out to be a significant part of an amp's sound, when in reality the difference in tube types (and how they affect the impedance curve) are subtle can be accounted for with EQ.
Yeah definitely. I have an EVH 100w Stealth (which uses 6L6's) and an EVH 100w EL34 and they're actually not quite but almost identical. WAY closer than you'd think from reading descriptions of them on the internet. However, the 6L6 is very slightly quieter than the EL34. There's a small chance that this could be volume knob tolerance but I've seen that more than once, so I kind of doubt it. I wonder if the people at EVH added slightly more negative feedback to the 100w Stealth, which would theoretically bring its 6L6 power section closer to the EL34's slightly more damped power section.
Bringing this back to the thread topic, I suppose Bogner could change the parameters of the Density knob to kind of account for the differences between the 6L6 vs EL34 power sections, but I wonder if they did. Honestly though, if your standard settings for the Uber Ultra will be to set the Density knob somewhere in the middle, chances are you could make the EL34 and 6L6 versions *actually* sound identical.
I suppose the other question to ask there, because I honestly don't know, would be "how does the KT88 compare, what is its natural damping factor relative to EL34's and 6L6's, and how would the KT88 interact with the Uber Ultra's power section controls?"
Just thinking out loud here but my original plan was to get the EL34 version and put KT88's in it, but now I'm not completely sure. To me, an "ultimate, modern super high gain amp," like the Uber Ultra is trying to be, should be very wide sounding, with as much usable bandwidth as possible, so tons of clear sub lows, clear mids, and crispy, presency high highs should all be well represented. It seems to me like the best choice for this kind of sound is either going to be 6L6 or KT88. I have to wonder if the KT88 will have bigger, more pronounced sub lows and more prominent higher highs than the 6L6, or if the opposite is true, at least at non-poweramp-saturating volumes. Also, I wonder if the EL34 version loaded with KT88's is going to sound appreciably different than the pure KT88 150w version if, again, the volume is set so neither power section is clipping.
