I don't know much about electronics, but I wonder if perhaps you had a surge that messed somethings up in your amp. Are you running it directly off your wall? Perhaps some knowledgable people here can step in and assess my theory. Just seems weird that everything was good and then all of a sudden not, especially since you ruled out the tubes. In any case, perhaps getting a decent power conditioner maybe something could help out in the future.
Good luck, I really feel your pain.