I have what I can only assume is a ground loop hum in a newly installed subwoofer amp. All of my equipment to date (mostly Sony stuff) has had 2-pronged plugs (no grounds). I haven't had any trouble with it at all. I took an old 15" DBX subwoofer I had laying around and installed a new 270 watt plate-type amplifier in it. It has a 3-pronged plug.
When I first plugged the thing in the hum was so loud that you couldn't even hear any of the signal (music, tv, whatever). I narrowed that down to the cable TV antenna. If I unhooked the CATV the hum went away. So I put a ground blocker on the antenna cable and it was cured. No more hum - but this was at my 'test' location where I could plug RCA cables from the sub-out on the amp into the RCA jacks on the amplifier.
Happy to have solved the problem, so I thought, I pulled speaker wire 30 feet across the room and put the sub in the corner (opposite another sub at the other end of the room). This time I'm using line-level inputs. I hooked them up and there's a hum. Not nearly as bad as before I blocked the CATV ground but it's noticable. I unhooked the cable TV. That's not the problem.
I have a 'cheater' plug on there right now which I know is not a good thing. The 'cheater' plug fixes it so I figure it's got to be a ground loop. The sub amp is plugged into a different circuit than the receiver. So I ran an extension cord over to the same circuit. Still hums. Then I plugged it into a 3rd GFCI circuit. Still hums. So I'm not sure what to do with it at this point.
I ran a wire from the back of the reciver (on a lug labeled 'signal ground') to the CATV splitter (after removing the ground block). Still hums. Then I ran a wire from the same lug on the receiver to a water pipe (which is where my electric service is grounded). Still hums.
Unless you want to call in an electrician to rewire the circuits from the service panel, I'd stick with the cheater plug and hope no lightning strikes find your house.
I ran a 3-conductor extension cord to the same circuit the receiver was plugged into.
I installed the service panel myself anyway so I could rewire it. It's actually a subpanel off the main panel that feeds my basement rec room. All the circuits down there are fed by the panel which I know is grounded/wired properly (since it's in the same building as the main panel it does not have it's own ground - it gets it from the main panel).
I'm just not sure what would need done. I've read that the hum is caused by the equipment finding different paths to ground. Right now the receiver isn't grounded via a 3-conductor cable so I don't know where it's getting its ground from. The amp is getting its ground from the service panel, which ultimately gets its ground from the water pipe. Theoretically, I think, by grounding the receiver to the water pipe I should fix it. I had an admittedly questionably connection there when I tried it. Maybe I'll get an honest-to-goodness clamp for the pipe and see if a better connection there helps.
I wouldn't ground to a water pipe if I were you. It is now against code in many areas to begin with. Secondly, it does present a safety hazard as evidenced by the electrician here in Dallas a few years back who lost his life while trying to source a problem and crawled through a puddle of water on the ground.
You would be better advised to install a grounding rod and tie all equipment directly to this rod. It will pay off in the quieter background noise of your system and the safety of a secure ground path should a surge come through the line. Think "star grounding" when you install the new ground path.