This statement was taken from this link to bluejeanscable.
http://www.bluejeanscable.com/store/subwoofer/index.htm
"Calrad Isolation Transformer:
There are two primary reasons why an otherwise healthy subwoofer may hum, and cables can deal only with one of these: EMI. The other possibility, a ground loop, is something your cables can't fix, and has to do with differences in ground potential in your system's power wiring. Although the best and most permanent way to eliminate ground loop hum is by fixing the underlying grounding problem, another simpler way to resolve the problem is to insert an isolation transformer in your subwoofer line to break the ground connection between your source and your subwoofer. We carry a small isolation transformer for this purpose, from Calrad, for $25.00."
I solved my problem a different way.
NOTE: Do not attempt this without some form of replacing the ground connection.
I used a 3/2 reducer plug on the sub. It eliminates the ground prong from the sub. I then connected my sub power plug to my line conditioner. My line conditioner is connected to the wall outlet by a 3 prong plug. I connected a ground wire from my line conditioner to my receiver. This makes anything connected to my receiver by a patch cable share this common ground. The sub is now grounded by the patch cable connected to the receiver, and there is no hum.
I also use an isolater. It solved my problem the first time, but when I added more equipment, I picked the hum back up. So, I then used my solution above.
One more thing. Make sure all equipment is disconnected from power while making any kind of connection.
