Jeff, widely separated(almost to the side wall, or where it would be) QS8s on the back wall work well(sushi has that setup),although a side wall on the left would give more reflections for a "side" effect.
On the center, I simply don't like the horizontal MTM configuration in comparison with a vertical speaker identical to, or nearly identical to, the mains. The vertical will have better horizontal dispersion and a smoother transition from the mains than a horizontal MTM. There's been 2 or 3 posts here reporting on favorable results with a M2 center. The VP150 has a different TMMMT configuration which apparently is a significant improvement over MTM, but which I've never heard.
