NH, the hum was not a ground loop. The hum was also absent on 3 other amps and two receivers plugged into the same outlet. This issue was definitely related directly to the Halo itself. (Note that in my earlier statement i thought i had related it to a Denon source unit but that was actually ruled out since i broke down and brought my big Onkyo upstairs for use as a preamp.)
If the Halo unit was reacting to something from the power supply in the walls, then its design is flawed but i'm not saying all these Halos do this either. In fact, i wouldn't be surprised if some have it and some don't. I've read at least one other report on the hum but quite some time ago so unfortunately i do not recall which forum it was.
In any event, some problems have been reported with other Halo units such as this lengthy thread
on the new units over at The Spot. Concentrate on the posts by Paul Carleton and you will see alot of what i came across. I found this thread just shortly after i took the Halo back from auditioning during my search for answers.
Notice how his use of "power conditioners" did very little to affect the ground loop issues he was having. The ground loops seemed to just move around from item to item but in no way did these conditioners do what they are marketed for. Power from the walls is and always should be regulated by the onboard power supply of the unit. I'm not sure that any electronics piece comes without a power supply that doesn't have regulation onboard. Conditioners are useless. Surge and sag protection are different stories. Bren can tell you all about his pro experience with power from the walls and into the machines over beer until you are blue in the face.
If you continue to read further down, Paul believes he has found the problem (after talking with Parasound), not in the amp but rather in the Halo C1 controller. After reading through this, i started to get concerned about the build quality (back to the design word again) of these Halo units. That is when i started this thread and since gave up on buying the A23.
I now own the Anthem MCA.
Paul summed up my own thoughts quite nicely:
It wasn’t bad but it would be nice on a unit with this price point not to have it at all. It can get pretty bad if you don't have the cable tie that Parasound is now installing on later units. Other than redesigning where the balance inputs go or where the power supply is placed, there is not much else you can do about it.
Again he is referring to the controller but i believe something similar is happening in the amp. I never did take off the lid to see as i knew the unit was going back to the store.
Incidentally, many others have reported no hum whatsover with their Halo units and enjoy them tremendously. If you have another amp, put them together and run them through the same preamp w/o anything playing. Turn up the volume and see if the Halo gets 'louder' either in itself or through the speakers. I had not realized the hum was abnormal until i had it compared to another amp and again i must stress that it was not audible during music playback.