Please clarify this.....
'none of the knobs change the sound, volume or input. Overdrive channel or reg.'
- Do you have sound, but are unable to change it?
- Is there no sound coming from the amplifier?
As Lt. Bob had noted and I have verified from the M80 schematic the 'power amp input' jack goes straight to the amplifiers output stage and there is no ability to control the sound as you have described.