[JThiessen]

Generally speaking, the advice given so far is on the money. A powered sub with built in crossover is the way to go for your fronts. AND you did the right thing by getting an extra set of towers for your rears (I assume they are identical to the RL and RF?). Timber matching the complete set is the "proper" way to set up surrond sound. And no, just haveing the same brand of speakers does not mean they are timbre matched. They need to be the same size drivers and performance specs to be numerically matched.
I run 4 Definitive Technology Mythos 4's with a Mythos 8 center and an M&K 1000W sub.