To my ears, the VP150 is a VERY detailed and clear-sounding speaker (although I don't have any other center channel to compare side-by-side). So it is difficult for me to imagine the possible reasons for the "muffled" sound you described.
One thing I can think of is the so-called "extended baffle" effect -- when you place the center channel on top of a TV so that it is flush with the front surface, the screen will act acoustically as an extension of the speaker baffle. This sometimes leads to an overt exaggeration of the upper-bass/lower midrange frequencies, and hence a loss of clarity/crispness.
I am sure you've confirmed that your VP150 is properly operating, but double-check that both of its tweeters are working at equal levels.
