have you tried using the clean in a different slot? first rule of troubleshooting: isolate the problem. also if you are using any pedals or external units...remove them and just plug directly into the amp. i think i know the sound you are talking about...though i don't know what it can be off the top of my head. make sure you have the problem isolated and then we can move forward. also check the bias of each tube and make sure they are within the recommended range (listed in the manual). also make sure that the impedance switch is on the right value (according to your speaker load) and that the voltage selector on the back is set correctly (100, 120, 220, or 240)