Loud hum from subwoofer PSW505B

[viperrcr]

I just purchased a new Polk PSW505B subwoofer for my system.
When I plug it in and turn it on, there is no hum. I hook up the Belkin PureAV subwoofer cable from my receiver to the subwoofer LFE, the subwoofer hums pretty loudly. The hum is adjustable with the volume knob. Funny thing is, when I turn on the receiver, the hum goes away completely and it sounds wonderful. When I turn the receiver off, the loud hum comes back. The volume knob is set to below 1/2 way. My receiver does not have a crossover setting, the sub gain setting on the receiver is 0dB and is turned on.
I have tried a few things such as lifting the ground with a 2 prong adapter, and the hum still exists. I unplug the sub cable, the hum goes away. I tried plugging into a different outlet, no difference (although it is in the same house circuit). The sub is plugged directly into the wall. I tried plugging it into my Belkin PureAV power conditioner, no change.
My other thoughts are: somehow this Belkin PureAV subwoofer cable is bad or I could try a isolation transformer. I do have an HDTV system and use digital cable (not analog). Maybe there is a problem with my old Sony STR-DE545 receivers sub-out? I was thinking of ordering an Ebtech Hum X, but they are kinda pricey.
The audio quality had always been perfect before, although previously I never had a subwoofer. Never experienced any hum in the other 5 speakers or any other component.
The easy solution is: I can leave the receiver on 24/7, but I'd rather not.

[apphd]

When you say "digital cable" do you mean an optical or coax cable, or do you mean from a cable service provider? First guess is a ground loop problem which lifting the ground would fix most times. For sh**s and grins if you were referring to a cable service, disconnect the feed into the cable box and see what you get.

Yes try a different sub cable and/or even for a test use the speaker level inputs and run from AVR to sub, then sub to a speaker, set the speakers to full range and see what you get with that. Once you can isolate what is generating the hum your 90% of the way there in eliminating it.