Actsreus, I still think whatever Al says is good advice. But try this short cut. Switch speaker cables left/right. See if the fading problems moves. If not, in some goofy way I do not understand, it could be the speakers. If yes, moving downstream, check the bias of the power tubes on your amp. Btw, how many hours on the tubes? If your pushing to the outer edge of the envelope on tube life span, I'd be looking to retube regardless. After these quick checks, back to what the folks above said.
Any problems I've had with my gear -- which have been few -- were tube related.