For some reason the tube channel is incredibly noisy. This is extremely noticeable when using the amp's DI out jack. Neither ground setting gets rid of the hum or whatever the noise is. I would like to get some idea if this is common, or what the problem may be before spending big bucks to have it fixed. Its a 1997 model. Replacing the tube with a Sovtech 12ax7 did little good. HELP!