Well, I think you may be expecting Logic 7 to do exactly the same sort of BS that has made me hate every surround processor for music since the beginning of time.
If you look into the Lexicon Logic 7 technology, it's really intended to recreate the sense of listening to musicians perform - in front of you - in a room. The center and the rears serve this purpose. I think you may like it better with Logic 7 off, because the rear speakers will be louder.
If you have no center, and your rears are set up as rears and not "sides", then Logic 7 may not be doing anything at all to your sound. Theoretically, you can get some benefit from having sides only and no center...but it still won't make the drum kit fly around your head or anything.
You may want to read the Wikipedia article on "stereophonic sound". It conveys better than I, the goal of a stereo system.
Musicar Northwest Portland OR musicarnw.com
High-performance audio for BMWs