Try cleaning the FX loop send/return jack sockets. These can cause problems in many amps. The jack sockets are "switching" types & the switching contacts can get a bit grimey over time leading to problems like this. Another possibility is that the "cathode follower" (aka tone stack driver) tube may be on its way out (V3b). This runs a kinda high "heater to cathode" potential & some new production tubes don't last long in this position. A common sign of this tube failing is a loss of volume. It may possibly be neither of these things, but it's worth a shot. Hope this helps. Cheers
