I did remove the coax cable from my receiver, and that freatly decrease the hum from REALLY bad, to noticeable.

That, was step 1.

After that, using an extension cord, it plugged the subwoofer into two other different circuits, but that didn't work.

The only thing that did work was to plug in the sub WITHOUT a ground. Knowing full well that that is not safe, I'm looking to get an isolator.

Getting one, at a reasonable price from a Canadian distributor, would be ideal.