I went with a pair of these:
Attitude 6.5" Woofer I put a 4,000Hz low-pass filter on them to help keep the highs anchored up front.
If I had a lot of subs like you, I wouldn't have done it.
But, my Focal components up front are not made for bass (I'm using a 120Hz high-pass filter on them), and I'm currently using only an 8" sub in the rear deck. I needed a bit more mid-bass kick, and they do the job much better than the stockers. Plus, for about $45 for speaks/x-overs/shipping, I figured why not.
I guess what I'm saying is that I wouldn't invest much money (or maybe none at all) in rear speakers.
If you're using coaxials (180w Pioneers???) up front, get some components (toss the Pioneers in back). If you're using components, power them with a moderate amp. Those are the places where you'll probably see the most improvement in your system.