You'll probably hear some varying opinions on this. Fact is the a vast majority of the dialogue in a movie comes from the center channel. So just looking at that, one could say the center is the most important. Most will tell you that the optimal situation is to have the whole front stage timbre matched and that the speakers should have identical drivers. It's not like the stuff won't work if you hodge podge it all together, but just sound quality wise- that's the preferred set up.
So, if you want to upgrade the center while purchasing the package, that is okay, but try to get a center that is matched to the front left and right channels. And have a blast!
So, if you want to upgrade the center while purchasing the package, that is okay, but try to get a center that is matched to the front left and right channels. And have a blast!