When I tested the Line-In cable, I reached around the sub and just unplugged it, holding it about an inch away from the line-in connection. Unless sound can jump an inch through the air, I would say that is not it...
I have a dedicated 20amp breaker using 12-2 wire for my HT equipment, so the only other items would be my AV equipment. All the power was shut off by the main switches for everything except the amps that I leave on all the time.
I can test some more later this week. It is just strange that it was so much louder prior to me powering off/on the sub, now it is pretty hard to hear unless your real close, and even then it is very very faint now.
