So I purchased a used Neo Pak a few weeks ago. It was working great up until last week when I plugged my bass in and got no sound out of the amp. Through some knob twisting I figured out the only way I can get any sound out of the amp is if I crank the Tube Gain and Tube Volume to 10. Then its a very low farting noise. Before I take it in I was hoping someone could give me a clue on what may be causing this? Thanks!!