If you can find a schematic, either post it or a link to it here so everyone is on the same page.
This is just a WAG, Wild Ass Guess. I'm assuming the circuit is basically the same as the RG-100 guitar amp that has been around for like forever. The input transistors of a typical solid state power amp can be degraded if they are over driven too hard or too long. It's usually reverse base to emitter breakdown. Many power amps have no protection on the inputs, driving them with a tube preamp can kill them. The transistors will become noisy before they fail outright. So carefully remove the existing transistors and try to find exact replacements. If possible, I would install sockets.