That's how my Superlead is. In the same room, some days it sounds like VH 1 and & I'm happy and other days it sounds terrible like I have worn tube(s) or something. The tone will be off and the gain just won't seem to be there. Very frustrating. I guess it has to with humidity, temperature, our hearing. I can't see how a fluctuation of a few volts from the power source would make an audible difference. Dirty power I can see making a difference though.
I plugged my amp into a variac and lowered the AC voltage from 122 down to like 118 or so and it made no difference in sound.