Just today I swapped the center with a 683 and it made quite a difference. Voices actually sound natural, and very crisp/clean..It was easy to understand even quiet dialogue and I did not feel annoyed trying to sit through a movie scene. At the moment I have no audyssey configured and as such that was a test using the same AVR, AVR config (no audyssey), and cabling, etc, using those two speakers.
I agree with you, becuase of the muffled sound from the cmc2 I ended out swapping out my cmc2 as well as cm9 HT setup for B=W CT series. that solved the issue for the same proce point. I listen to 0% music in that room