Kim,

I have QS8s for rears and sides. I am convinced about the QS8s as sides but not as rears...at least not if you are sitting 13 feet away from them. I've wanted to try direct radiating for rears but I haven't had the time yet. So my advice is that if you plan on sitting far away from them, examine your room geometry and definitely consider experimenting with direct radiators and reflectors as well. M3s would be a good bet for rears.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated