Agree with Shady. An identical bookshelf is about as good as it gets for timbre matching a center. If you want to bump up your center you should really match all 3 up front.
If you like your SM55s and feeling an upgrade itch the SM65s are pretty nice speakers and you might be able to find them at a discount since DefTech launched a new line. Three SM65s would make a nice front stage and you could move the 55s back for surround duty.