I've had problems with wall power before (I'm in Florida, where the wall voltage is 110ish, not 110). If your amp has a sweet spot, and the wall voltage goes up or down a bit, everything else can go with it.
I put everything on an isolator (not just a conditioner, but one that puts out a constant voltage no matter what goes in) and my tone variations disappeared. That's a pretty drastic measure, but I happened upon a Liebert unit for nothing. A "filter" won't do much for this--it'll take out stuff that's not 60 hz, but if 100 volts go in you'll get 100 volts out.
I didn't notice much of a change in my clean sound when the wall voltage changed, but it REALLY changed the crunch sound. I got a little wall voltage meter for Radio Shack and stuck it on the same line with my gear and that's where I really noticed it.
You could also have a heat-related failure. Does it sound good when you first power it up, but gradually fizzle out over the next half hour or so? If so, there could be a part getting ready to go. The part will work when cold, but start to crap out as it gets hot. Sometimes these go unnoticed on a bench, when the unit is turned on but is not inside the case (it cools better).
