newbie with speaker selection ?'s

justlernin
justlernin Posts: 2
edited November 2007 in Speakers
I am upgrading my system from a 5.1 to a 7.1 surround home theater. Room size is 15'x20' and open to an additional room of 15'x15'(est).
My A/V is an ONKYO tsxr875.

I currently have Polk rt2000i towers for the front, Mb quart center channel,Polk f/x300i surrounds.

I want to add a subwoofer and rear surrounds. So my question is: which speakers would you recommend to compliment the older units that I have. And also should I consider replacing the center channel speaker? I have read that it is very important to match the front sound stage, and I honestly don't know if the MB quartz, although high apparent quality, is a good match for my system.

Any help or advice would be apreciated.

I have edited this post to pose another, possibly more appropriate question.
What is the criteria for matching speakers, specifically the sound stage? and can this be done by viewing specs on a particular speaker? Or does the matching require specialized audio equipment?