AVS Special Member
Join Date: Dec 2007
Location: Southern California
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Having the subwoofer plugged into a separate circuit may be the problem. There may be a grounding issue there.
Try plugging the subwoofer and receiver into the same circuit and see what happens. Do not use the surge protector either; set it aside for the test. I assume that you have a grounded 20-ampere circuit.
Also, I would suggest that you use the line-level inputs on the subwoofer, not the speaker-level inputs. There can also be a grounding issue there.
You will also get better performance if you set your main speakers to large and use the subwoofer's own rolloff control to limit its output so that it only operates below 60 Hz. You do not want it operating at higher frequencies.