A lot of good information in this link for you.http://forum.blu-ray.com/showthread.php?t=97963
Generally, a ground loop is formed when an electrical system has more than one path to the electrical ground. The path between these points form a loop that works like an antenna and picks up noise. Grounding problems are normally found because you are using 2 prong and 3 prong power cords and plugging them into different wall sources.
You need to find the source of the problem. The only way is to unplug them one by one to see what is causing the hum.Usually, the most common sources are cable TV boxes and computers that are connected to an A/V receiver. Unplug the cable box and/or the computer and see if it improves the hum.
Sometimes, changing the wall outlets solves the problem.
Don’t run interconnect and speaker cables parallel to power and extension cords.
Don’t coil up the excess length of a cable. If you have excess cables, instead of coiling them, you should make a figure 8 with them.Use an RG6 digital coaxial subwoofer cable and move it away from the other cables. This may help.
Buy a good power conditioner/surge protector and plug all your equipment into that one, including the subwoofer amp. Make sure the coax cables for the cable TV also go through the conditioner. This may solve your noise problem.
Buy a ground loop isolator.