While reading your post I had a strange thought. Is there any chance that one of your speakers was ever repaired and connected backwards to the terminals? Try wiring one of them out of phase and see what happens. I did this in college to a neighbor that would run his stereo early on Sunday mornings playing video games. It was on the wall opposite my bed so I took things into my own hands. It almost sounds like you're in the situation that I put him in.
Also, your Washington and Manchester made me think that we were neighbors, but I'm actually on the opposite coast.
Also, your Washington and Manchester made me think that we were neighbors, but I'm actually on the opposite coast.