The cable box was the first thing I disconnected but, as I said before, that wasn't the problem as Rex and Swampmaker suggested.
Unfortunately, with all the high-power components in the system (amps, powered subwoofer, TV and several transformers/conditioners), it is not possible to hook everything to the same AC outlet as Riffer recommended without overloading it.
I guess I will have to find this gremlin the hard way: float everything except one common ground (processor) and check every single AC polarity for the lowest noise.
Any other options?
THANKS.