Are your speakers in phase? Are the cables in phase. Is there an inverting switch somewhere?
I didn't read through the entire thread but this is exactly what happened to a friend of mine. He opened the cabinets and found one of the woofers had the polarity reversed. Damnedest thing I ever heard, but it made sense after we put all of the clues together-our position in the room, the musical content.
It was one of those unexpected issues, if you know what I mean. He thought outside of the box (or inside, in this case). I would have never suspected the woofers came from the factory wired in reverse.
It was a simple case of wave cancellation.
(Sorry if this was discussed, I didn't feel like reading 10 pages of this thread).
Rich