I think you need to flip the problem upside down; ie, think about how to mount the center channel speaker on a shelf above the TV rather than mounting a heavy TV on a shelf and placing the center below it. Anything is possible, but some things to consider...
- Mounting a TV on the wall requires a large bracket secured to multiple studs, which in my house is not so easy given non-standard stud spacing on the wall I'd place it. Using a shelf would present the same problem and in fact a much bigger one.
- To secure a shelf that is wide enough and deep enough to hold a TV will require far more than just bolting it to wall studs. You'd also have to bolt some kind of header below it as well as use L-brackets to support the ends. It will not be stable enough without brackets to support the part jetting off the wall - the shear forces will rip it right out of the wall. It can be done but would probably look like an eyesore.
So...consider leaving the TV on the stand and mounting the center above it on a shelf. However, you run into the exact same issues if the shelf needs to be very wide and very deep to support a large center. It won't weigh anywhere near as much as a TV but still needs extra support.
I thought about and abandoned both ideas (for one thing you cannot buy a pre-made shelf that is anywhere near decorative in the dimensions needed - it would have to be custom made).
My TV sits on a stand I had built and the center goes below it in a shelf in the stand. I've had center channels both above and below the TV at various times and there really isn't a huge difference. Sometimes its best to just keep things simple.
