Well, if the amp still wavers in volume with another preamp plugged into the return, the problem is isolated to the effects return recovery gain stage and the power amp. If the problem ceases, that isolates the preamp, including the effect send circuitry. But you said the volume variation occurs through all channels of the amp, so that would indicate that the problem is in the effect send circuit, as that is common after all the channels.Ok I’ve bit the bullet and ordered some new power tubes to test with. I can only hope that’s the issue. They arrive either tomorrow or the day after - I will update you guys once they’re in.