There's one other center speaker configuration you might want to consider.

I laid two M5HPs horizontally so their HP woofers were adjacent to each other. Then I laid the VP160 on top. In total I had four woofers, three midranges and 3 tweeters. They were all wired in parallel and powered by a channel off the ADA-1500. Then I re-calibrated my system. Booyah! I love the 160 and 180 but this...this is so freakin' amazing!

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated