Either this, or perhaps, you have lost a valve in your poweramp section.
Turn your amp on, and leave it on standby...then take a look at your power tubes and make sure all of them light up to the same degree, if one is not lit...that is your problem...other than that I am not sure what it could be.
[FONT=Century Gothic]Hope is when we feel the pain that makes us try again[/FONT]