OK, so I finally found my bass - but I have no idea why it builds up where it does or what to do about it. I had already discovered the first part of the bass buildup and put a trap there (3 sheets of Roxul ComfortBoard vertically across the wall corner), but when I found a longer playing bass track and went further into the bedroom I found even more bass.
Rough floor plan below. The area to the left (above speakers & listening position) is open to a cathedral ceiling, while the center section is under a loft (beams & 2x6 T&G) and the right section is ground-floor only with conventional trusses above. The primary bass buildup is a few feet to the right of where the loft starts.
Only clue so far is that the cathedral ceiling is in sort of a cross shape above the area with bass buildup -- will try to figure out how to photograph or draw that.
EDIT -- here's a pic with details of the loft. Listening position in the following pic is for the HT area, not the stereo system in the living room on ground floor. Also the piano did not "go there" in the end, it didn't really fit up the stairs so it's still on the ground floor.
I guess the bass is building up under the peak where the wall and ceiling raise up. But why there rather than under the peak WHERE I PUT THE COUCH AND THE LISTENING POSITION ?
Stupid bass...
At least I may have found something worse than a square room
