Hi MikeG,
The vagaries of ground loop hum can prove mysterious and very annoying. You seem to have tried almost everything. If you disconnect the low-level coaxial cable at the subwoofer input, then turn on the sub, is there any hum from the subwoofer itself? If not, then I still think you may have a ground loop. If there IS hum from the sub with no cables connected except AC power, then there's something wrong with your subwoofer amp.
Did you disconnect the cable-TV feed at the point it enters the cable box? Try that. If the hum disappears, then it's a ground loop from your cable system, and you need to order the cable-TV isolator from Axiom. If you still have hum after disconnecting the cable-TV feed, then I suspect there may be a defect in the Onkyo.
Did you try disconnecting all the inputs, audio and video, from the Onkyo, then listen using the headphone output. If there were a defect in the Onkyo amp or preamp section, you would still hear the hum. If the hum goes away, then it's a ground loop somewhere in your system and not the Onkyo.
Your AC power supply from the wall outlet cannot in and of itself cause hum; there has to be a difference in the grounding of other components, including the cable-TV feed, which is almost always the culprit.
Let us know the results.
Regards,
Alan
