I like wall mounting the TV, getting the screen about 12" away from the front baffle of the Center...
And getting any Center out of the Cubby:
Sin!
I also agree with TLS, but will stop short of telling you to buy a new center.
However, I do always like a 3-way Speaker and center is no different.
(If you wall mount, you need to make certain you are going in to Studs. Don't buy a wall mount that does not allow for horizontal adjustment.)
Also, you can buy legs for the TV that screw into the VESA mount on the back of the TV... some of these may be tall enough to allow for your Center to be above the av cubby...
SIN!!!!
Sorry... I really... REALLY dislike placing Speakers in a cubby.