Hi, I just picked up a demo model V-4B reissue off of sweetwater and there is a hum coming from it. Dead silent when in standby, only very slight hum with nothing plugged in (only audible when gain and volume are dimed). Plug in a cable to the input with no instrument and the hum gets louder. Plug in a bass and it gets even slightly louder (the worst with my Rickenbacker 4003, but that has to do with the high gain single coils I would imagine). Any ideas of what could be wrong? loose preamp tube, bad power? Should I just return it? I love the the tone of it so much up until I noticed it was humming a lot.