Polk Audio - How To Choose Center Speaker?

In a few areas of my home, I have a 3:1 or 5:1 AV setup.  I was planning to purchase Polk Audio RC85i in-wall speakers for my front left/right.  However, Polk Audio's website doesn't seem to have a matching RC line center speaker.  My center speaker can not be in-wall and I plan to mount it right below my tv's.
 
I've read that it's important to "match" the correct center speaker with the other speakers...
 
Thank you in advance for your ideas/thoughts...

I have tried three different center speakers.  First one matched left and right speakers, second one didn't and third one has worked just fine for me.
 
It is the Bose VCS-10 which is now discontinued.

Polk website Q&A suggests the "TSX150C or the TSX250C.  That will be the best match for you." -Polk_Support_Ken
 
http://www.polkaudio.com/rc85i/d/1188

If you are handy, you could build a box for a third inwall... Wouldn't be too tough.

Thx everyone & Neurorad for the tip...  Never thought to read the Q&A to find which speakers match...  
 
@mdesmarais - there are too many studs in the way where we want the center speaker (not to mention that we have a custom cabinet) so going with the TSX250.