I had the same problem with a ground loop between my house ground and the cable ground. I put a 3 to 2 prong adapter on my home-make amplifier and the hum was gone. The cable box is now the sole ground in the system (through the coax cable). The rule of thumb here is to allow no more than one external ground. I have never seen any commercial HiFi components using the ground prong in the power plug. Components are usually designed to "float" with respect to the mains. If the hum persist but goes away when you unhook the cable line, there's a trick to isolate the cable box from the cable ground. Take two balun transformers (the adapter people used to use to hook cable up to old TVs that took twinlead) and connect the twin lead outputs of one to the other. Now you have a 1:1 RF transformer that breaks the ground loop. I would only try this as a last resort. Now that cable is a two-way digital system, the baluns might mess up your cable reception. Bobby D.