After systematically turning everything off in the house, I finally discovered the source of an annoying hum that was most pronounced in the same spot as my favorite chair in front of the stereo/TV. I wasn't sure when it started, but it coincided (in retrospect) with another annoying phenomenon: cancellation of some low end frequency at head-height while seated in that same favorite chair. What rotten luck! I just couldn't believe I hadn't noticed this cancellation before, and until I had eliminated the annoying hum, I hadn't connected the two phenomena.
So what was it? It turns out the external USB hard drive sitting in my basement studio was the cause of the hum. Somehow the frequency of the hum from the spinning platters in the drive was sympathetically transferred upstairs via the surface the drive sat on, up the studio walls, through the living room floor boards and into the air... directly overhead to where my favorite chair sits. This hum's frequency (117 Hz, as I later determined) was loud enough to mix with the audio coming from the living room stereo, and caused destructive cancellation of that frequency -- a frequency well within the low end range that a subwoofer is concerned with, and very noticeable when it disappears. I was really pleased to find I had actually fixed two problems when I had only set out to isolate one. Make that three problems solved: sitting in my studio afterwards, I noticed that the room seemed quieter. I must have been subconsciously ignoring the hum down here. In any case, this was all a good reminder to listen to what your ears are telling you!