i searched for a while on this forum with no luck. i've got a non-pro svt 2 and it has started a very serious problem. after warming the head for ten or so minutes, i plug in and turn the stand-by off. an audible hum and a *very* weak tone from my bass comes through. then the right bass tone will come in and out for another five minutes and then the problem will finally go away. then, it's beautiful svt tone like i've had for the last six years. i changed the power tubes and one preamp tube that was broken about a year ago. i'm using the same 4 ohm cab. this thing is built so solid and all the tubes light up when it is powered, it has me baffled. any clue? i've got a show on thursday!