Ideally, all speakers (except sub) should match. But the rears aren't as critical as the front three (as was mentioned). Probably aren't very many of us that does have all the speakers matching. In the real world, things are seldom ideal. Your system might sound a little better if all the speakers timbre matched (if you could afford it).
Check out the SVS subs:
http://www.svsound.com/products-sub.cfm
Outstanding subs.........
