I don’t know if I missed reading it, or no one suggested it...

How about swapping the cable box with one of the other boxes? If the problem goes away, well that box is bad. This can go with the power block, that is if the power supply isn’t built into the box.

Also, since the sub and av are on different circuits. Is it possible they are on different panels too? If so, that could be a problem in itself. They could be on the same phase, but different panels. Thus the sub panel can not have the grounded (neutral) and grounding (ground) bonded together like they can in the main panel.

KOT