I recommend you read this link regarding surround speakers.
http://www.axiomaudio.com/archives/surroundsexplained.html
One thing to point out is when you mention rear speakers, I'm sure your referring to the left/right surround, there is a difference. Unless your talking 7.1, then you have the left/right surround, and the left/right rear speakers. A direct radiating (bookshelf) speaker will not envelope you in the surround sound like a quadpolar design speaker like Axiom Qs4's or Qs8's. With direct radiating you have to almost be in the sweet spot. The reason why theaters are lined with direct radiating speakers along the walls, is to surround you with the experience. The Q's have 4 drivers firing all in phase to give you an experience you will never forget
