Help with ringing bass from Dynaudio C1

I recently replaced a pair of Silverline SR17.5's with some Dynaudio C1's. I love the Dyn's, but one thing is driving me crazy: on some passages with a deep bass drum (e.g.,Young People's Guide to the Orchestra on Mr. Britten's Orchestra), the bass starts ringing, obliterating other frequencies and sounding very much out of control. I have tried putting an acoustic panel in back of the speaker. That helps some, but the bass is still resonating too much. It sounds like one note in particular, but I can't say for sure.

System:
PWD mkII via ethernet bridge
Bel Canto Ref 1000M
Audioquest Sky XLR, & Volcano speaker cables
Audience AR6T conditioner