I just bought a used Mesa Dual Rectifier 100W (3 Channel) head from Guitar Center. I've had it for about three days and I think I've got a problem with it. I'm using the head with a Marshall 1960A 4x12 cab. I'm running it stereo to the cab with 2x8 Ohm inputs. When I turn on the head, I usually leave it in standby mode for a few minutes to let it warm up. When I take it out of standby and try to use any of the three channels, I have to crank the volume to hear any kind of sound. The amp isn't giving me any kind of high gain sound at all - just a lot of hum/hiss and regular old guitar sound. No matter how much I crank the gain or fiddle with the EQ, I can't get the sound to change. I played the amp the day I got it and the day after with no problems. The day after I got it, I did have to cranked to about half way on the volume (I had it up pretty loud) but everything was working. No problems at all. Any ideas of what could be causing the amp to act funny now?