I used to have a SWR Bass 350 the chrome version. I was happy with that. I got it from a member here at TB for a good deal in like new condition. Sometime recently, I thought that I would like to have a SM-500 or SM-400S for the extra power and more versatile EQ. I went in to a new shop where I live and there was a SM-500 that did not look too bad. I worked a deal with that shop for my Bass 350, Ampeg B-115(older 70s to 80s?) and Fender Bassman 25 plus a little cash and took home the SM-500 with 30 day warranty. Before I traded the Bass 350 it had a hum which I thought was the preamp tube. I don't recall that it hummed that much when I first got it. I tried out the SM-500 in store and thought it sounded good and did not hear noticeable hum. During the past week it has bothered me more. The hum occurs with and without anything connected at the imput. I tried two different preamp tubes and the hum is definately not as loud now but still iritating. The hum gets louder as the volume or gain is turned up. It is present at the effects send. I connected a patch cable to the effects return with no signal going through it and dialed the effects blend completely right and the hum went away, all that remained was a very quiet hiss that to me seems normal. I do not know if hum is present at the XLR but I assume it probably is. None of the pots or sliders seem noisy. The hum is annoying even at a low volume. I tried changing speaker cables and tried all of the various outputs. I tried a different power cord. I tried it in various rooms and circuits in my house. I unplugged almost every other electric device in my house. Nothing stopped or decreased the hum. To me this is not normal and I really think that this amp should be very quiet. I would like to know what others think and what their experience has been. I still have 20 days on the warranty. I can get the Bass 350 back if he still has it but it hums too. I have done all that I would want to do moneywise with the tubes. I don't want to spend $ for a tech to look at it. I think that the only thing that I have not tried is taking the amp to another place entirely and see if it hums there. Should my amp hum this much?