I've had my 350 for a couple of years and have liked its 'sound' with its tube gain as high as possible without clipping. I changed the tube a few months ago and it worked fine until recently. I'm not sure if this has something to do with the tube, but you tell me what you think. MAIN QUESTION: When I plug into the passive input and turn the volume up on the guitar and amp, the volume goes slowly (every few seconds) up and down?? and if I watch the speakers the cones slowly push out and in when the note is playing. almost like the zero line of the wave is moving up and down?? after I'm plugged in for a few minutes its stops? if I run through my RBI (solid state) and return into the power section of the SWR every thing sounds good and stable. On a minor point there is a buzzing when plugged into the swr, but that maybe a ground looop from the DI wire plugged into the RBI output. any ideas?