After recent adjustments, on Sunday I though I'd go back to play "Spanish Harlem" sung by Rebecca Pidgeon on the "Raven" album. That has a bass line which has been said (by some famous audio engineer) to be good for tuning bass. The acoustic bass notes should be roughly equal in volume. (Actually, on most of my systems, despite having subterranian bass, I get the third note, the higher of the first three, to sound slightly louder and different in character, as if it's a plucked open string, though I'm not sure that's true.)
Anyway, now I noticed some very annoying rattling on that third note. Thinking it was a panel problem, I muted the acoustats. Now the bass line coming from the sub is transparent through the entire piece (that's all I hear on the sub) and the third note still rattles horribly.
I tracked it down to the fireplace mantle. ASAP, I'm going to start removing stuff from the mantle until I can make the problem go away. Now that this problem has made itself very clear, it's a good time to fix it.