MM, welcome. From what you describe it appears that, contrary to the usual situation, the difference in electrical potential at different grounding points(which is what causes the ground-loop hum/buzz)is even greater when the cable feed isn't in the circuit. Mysterious, but it indicates that putting an isolation transformer on the cable feed likely won't help. The outer ground shield on the coaxial cable should be isolated from the input on the sub so that only the center signal cable is connected to the sub. With the sub volume low, it can be tested by slowly pulling out the RCA plug to see if the center can remain in contact(to feed the signal)while the outer shield is no longer in contact, which should stop the hum. If it does, you can try covering the sub input contact with a piece of plastic film or strip of paper so that the outside shield doesn't make metal-to-metal contact, even when the RCA plug is fully inserted. If this works, it's free; items called ground loop isolators are available for $10-$20(such as
this )which connect between the sub out on the receiver and the input on the sub. Note that in either case the sub is still safely grounded through its 3-prong power cord. With the shield on the coaxial sub cable connected at only one end, it's slightly less effective at shielding outside interference but is still generally good enough.
