Having run 2 M3's as a center I would strongly suggest like some others have here already that you look hard at the M50's. They are seamless with the M3's.
The M3's and the M50's sound identical and thus you get great left to right panning and vice versa.
However, I should also point out that I have never heard the M40's but I still say if you can swing it, get the M50's.
When I went with MA, I continued to run dual centers for a fair bit of time. I don't now as I get equal results from my current center but I still like the idea of dual centers.
