After various searches here (TalkBass), I have not run across this particular scenario - but, cross-info speculation leads me to ask the following: So, a "rehearsal room" containing: A) computer w/recording software, B) pre-amp gadgets, etc. for microphones, XLR/quarter-inch "ins" for bass, drums (pads as trap set), guitar & keys that feed said computer, plus headphones that receive the mix (watch your step!), C) recessed ceiling incandescent lamps, D) security "smoked-glass bubbles", . . . can all create a "noisy" headphone environment? "Live", the bkgd. noises are negligible. For instance, my Hartke has a wee bit o' treble hiss, but . . . Headphone-wise, I'm the "culprit" > Ibanez BTB (active) > Hartke 210c (speaker unplugged; XLR out). My amp level is (maybe) 8 o'clock, my ground lift on/off makes no difference, my XLR pre/post makes no difference, adding/subtracting pedals - nada (usually, just a TU-3 tuner), kinda' fiddling with the detached 1/4" pre-amp/amp feed that goes to the combo speakers affects the bkgd. a little - but, there's a hiss in the phones - until I turn off the amp. But, when "on", I can move (walk, or sway) and the sibilance will increase/decrease. Raising the head of my bass toward a recessed light will increasee the hiss . . . Did I mention that it's a "finished" low-ceiling basement & we're stuck in a 10x12ft. section (double glass panel-doors, nice paint/carpet). It's the drummer's place (he's also the sound/record tech) . . . What's a good troubleshooting starting point? Sometimes, the drum set-up (pads and/or module), or the keys go wonky, but that's a much worse "crackly" intrusion (or complete cut-out) . . . And, of course, the others hate me . . . I'm going mad I tell you, mad!!!