Hi guys, I yesterday received a used GK MB800 head to power my SWR workingman's 4x10. I set it up, following the User Manual, put the master at 12 o clock and adjusted the gain. It sounded great initially, but a few minutes in, the signal faded out to nothing. Turned the amp off, then back on, and it worked fine for both an active pickup and passive pickup basses. Now I find that flipping the mute switch on will mute the signal, but when I flip it off again the signal does not return. Any ideas? Thanks!