I've got the S-85's as rears myself. While the S-85's are great direct radiating rear speakers, I've found that I much prefer M&K's tripoles for rear speaker applications.
Any of M&K's tripoles that I've heard are excellent rears. Be it the very little K4 or the SS-55's, or the SS-150's.
The one bad thing about the S-85's right now is that they've been discontinued. So you're not likely to find a new pair. The price on them new was $800 a pair. So as Larry mentioned they'd be a good bit cheaper than SS-150's.