I'm getting the occasionally volume fluctuation with my Genz Benz GBE 600. The volume sometimes starts to fade out, and then fades back in. This usually happens over a span of ten seconds. The clip light on the tube pre section fade down from the slightly overdriven state I normal have it, leading me to believe that it may be a problem with either the pre section itself, or something from my bass to amp. I have an effects board, but bypassing all the effects while this phenomenon is occurring doesn't have any effect. I'm thinking the cables may be at fault here. I also thought it could be a problem with the preamp tube, but I think I got it to happen on the FET section also. I did play direct into the amp for a few minutes and it didn't happen, but I didn't play long enough to tell for sure. Any ideas?