I have a loaner Shuttle 6.0 that I used a couple of gigs and loved, but in replacing the preamp tubes in my other amp, wanted to try out. Decided to use the Genz again for a gig last weekend, but when I turned it on I got some low volume crackle and pop stuff. It was happening even when the input was muted. I didn't have time to try much, so I just switched amps with same cables and cab, etc, and all was well. Opened it up and tube seemed seated well, and all ribbon cables. Haven't had a chance to hook it back up at home to check, but will try tonight. Oh yards, when having the issue, the bass sounded great still. Sound guy was affraid sound would come through out front or house, so for the sake of ease, swapped amps. Could this just be dirty, loose effects loop jacks, any other ideas?