I have all my theater stuff connected to a large APC UPS and have used APC UPS's for many years. Unless it's defective, I think it's extremely unlikely to be the source of a ground loop. Most of the APC units have an LED on the back that can indicate if it detects faulty wiring at the outlet it's plugged into. It's almost certainly a loop introduced by your cable box as that is a frequent culprit.

Cheers,