Ever since adding the 500 to my setup I have been battling a ground loop issue as well. I resolved it by adding an in-line isolator to my sub connection. I first tried one on the cable-tv but that didn't help. No more ground loop but I've always felt I was robbing my subs potential. Don't get me wrong, it performed wonderfully but it just nagged me.
Either yesterday or the day before, I was reading someone elses solution. He ran a 14 gage wire between the sub and his receiver. This morning I tried it. I took off the isolator, connected the wire to chassis screws on the sub and receiver then powered everything on. Although hum was still there, it was far lower than before. Next I unscrewed the ground loop screw on the back of the 500 and that took care of the hum completely. Prior to the isolator, unscrewing it would have made the hum twice as loud.
Excited with the results played an SACD disc but had to stop it after a couple of minutes. The bass was much to strong. I had to recalibrate the sub to compensate for the increased signal reaching it.
If you can, try it. Maybe you won't have to add an isolator after all.
