DON'T GET 6x9's!!!!!!! All it will do is cause bass cancellation, distortion, and the sub could damage the cone! Yes they can due to pressure from the sub air!!!
However, if you must get rear fills, get coaxial speakers, and place margarine tub underneath the cone (I kid not
ALSO, AMP YOUR FRONTS! Main priorety (if not done alreadY) you won;t get cancellation then!
Try amping front's before getting rear fills though..
Ray