Not that it sounds like you are going this direction, but I moved my center from under to over my 50" DLP and it did improve the clarity of vocals. I built a shelf from the wall that "levitates" over the TV by a fraction of an inch. The shelf is thinner than the center so it is not very obvious from the side, and invisible from the front (if the TV is in the right spot). The improvement was not dramatic, but noticeable improvement in clarity/detail at lower levels was obtained.

Let me know if you want detailed pics as I think I have the physics worked out (i.e., it did not fall or vibrate) but would suggest one change in the design.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire