Yeah, a single M40 could do justice however, there are some variables to consider. First the ergonomics; center channels are designed to lay horizontally and not stood up. Book shelves are designed to operate just the opposite. If you lay the M40 on it's side, it could work but you're not optimizing the dynamics of the speaker.
Second is, the M40 carries 5.5" drivers, so do the M60's. The M70's carry 6.5" drivers, same as the CS2, so IMO, you could use an overpowered center with towers, but not an underpowered center with towers.
Now, with that all said, hey, if you knew what I've started with from the beginning, you'd know I'm a Dr. Frankenstein at heart and I've gotten by with some pretty make shift operations when it comes to H.T. I didn't always have an AVR and 5.1 or 7.1 set ups.
I took an older Fisher CD player that had detachable 3way speakers and had pre-outs on the back. I took a pair of cheap sony book shelf speakers and spliced RCA connectors to them to hook them up to the CD player. I ran video to my TV and AUX to my BRP and presto, I had me a total make shift 4.0 HOME THEATER system The CD player was even hidden in a cabinet with the speakers on the wall, with no remote, I had to open the cabinet to adjust the volume, my, what a distance I've come
So yes Grasshopper, you can use the single M40 for a center, but when you can snatch the HDMI cable from my hand, it will be time for you to leave the temple.