Hi jonjons,
Hmmm. Very interesting. Try disconnecting your coaxial line-level cable at the subwoofer end right after you've been running the sytem, so the subwoofer amp hasn't yet cycled off for lack of a signal, and see if you're still getting the hum. If you are, call Joe Vassallo, Axiom service manager, on our toll-free line and he'll take it from there.
However, I'm almost positive it's a ground loop. Have you tried reversing the AC plug--if you can--on your receiver, and then listening? Or re-connect the line-level coax to the sub and reverse the AC plug on the subwoofer. Do one or the other, not both, and see if the hum disappears.
Regards,
