It's either a set up issue, a faulty center channel speaker, or faulty center channel output on the amp. First, we have to rule out set up. Turn off Audyssey and set all channels to the same level. Now play some content back: does it still sound muffled? If not, Audyssey was screwing things up. Now with Audyssey off and all channels set to the same level, we need to run test tones on each channel individually. When you run test tones to each channel, they should all sound similarly loud. If the center channel is quiet, then that could mean it's a hardware problem. Then what you do is swap one of the left or right speakers for the center channel speaker on the amp. If the speaker on the center channel amp channel is still quite or muffled, then we know it's an amp problem. However, if the problem follows the center speaker, we know that it is a speaker problem.