Sounds like you lost yer power valves. The sound gets all crackly and nasty ... like the sonic equivalent of someone giving you Doritos when you ordered nachos ... crunchy, but in a bad way.
In my Rivera, that amp would work no matter what condition the power valves were in, but the sound would just get awful. When I pulled out the power valves one time because the amp sounded like butt, there was nothing left to the heaters in two of the valves! They had completely disintergrated!!