If you have the space, the money and the will - the 5ks at the back would really be the way to go. Probably more noticable in a largish room - or at high volume.

I've only heard 33's at the back once and I wasn't too impressed - they noticably lacked the depth of the fronts. (Starwars Epi. II Demo, 5.5k's F/C/R, 33's at back)- this was in an open-ended room though, and they only had the 33's hooked up as they had sold their demo 5k's.

My 2p's - No refunds!.

Rob