I did pull the woofer last night, but I did not see anything amiss (not that I would recognize a whole lot, but the wires were securely attached and the woofer appeared to be solid and in one piece, no rodents living inside the cabinet, etc.). Incidentally, I had no problems with magnets, I guess because the Emerald is magnetically shielded.
Put everything back together, but no change in sound. As I was listening, it occurred to me that I probably am mis-describing what I'm hearing as resonance, but I'm not sure what else to call it.
In any case, it's time to call ACI. Thanks for the suggestions,
Mike