Couple things happening - when the audio/video feed changes from HD to regular or visa versa (using Comcast), my Sub POPS loudly. Second, when the receiver is off the sub sometimes has a constant loud humm noise. I have a JBL PB10 Sub connected to Pioneer Elite Receiver. I have settings: Level set to middle, Phase 0, LFE and connected using sub cable into Left speaker input. Any thoughts on these issues ???

popping well thats probably fault of cable box switching audio streams/chan + receiver re-detecting audiostream (mine doesnt pop loudly... sometimes a small click)

hum = probably ground loop = oh boy alot of work and headache to fix.

(try removing every input from the receiver, and outputs to other devices-tv, if still hum probably from electric source. else start pluggin things back one by one)