It might also be an effect of the increased mids in the voicing of the 6534+ relative to the 5150 II/6505+. I let him play through it, so that I could stand back and listen and really compare them being played by the same person, with the same guitar and cabinet. Even with the EQ at the same settings as his amp, i.e., really scooped with a heap of presence and resonance dialed in (in fact it seemed to be pretty much dimed except for the mids, which were on 1 or 0, and the pre-gain and post-gain, which were on 7 and 4 or 5 respectively) there was a noticeable increase in mid-range cut vis-a-vis the other guitar (another Schecter through a Krank Krankenstein head and Revolution cab). Could this middiness also have contributed to the impression of 'tightness'? It seems to be the lows that people associate with 'looseness' or 'flub'. This might also be a product of the 6L6 power tubes as much as any changes in pre-amp circuitry (pre-amp tubes are the same type, although there may well be different brands installed). I think he had the tubes biased recently, but to me it didn't sound much different from before. His amp always sounds like a bulldozer or something, very big bottom end.