Sorry, Doc House, my previous post was being typed as you posted yours. I seem to have some sort of Cable TV-to-receiver via HDMI squiggly-amp conflict since yanking the Shaw box off the receiver cured the hum. Electrically, the Shaw STB and the Pioneer receiver are on two completely different circuits of the house, fed via two completely different UPS's.
I cannot imagine how the jack could be damaged. The sub came in an undamaged box and has not been physically assaulted by anybody/anything since it arrived. I simply moved the sub into position and plugged the RCA into the adapter, then put the adapter into the sub, then slid the sub back into its position. The jack/recepticle behaves much better when the jack oriented vertically versus horizontally. I just made a video of what I mean:
Dave's EP350 Input Jack Problem
