I have 850s on the front and LCR55 at the Rear. 2 Questions:
1) which is better S85 or 850
2) Will the S85 be better rears than the LCR55

thanks

You tell us, you've had S85s so you should know.

M&K 850 speakers are much better for music, but the S85 especially with it's dual crossover selection is better for movies. That's my subjective opinion, I've owned and used the following:
S-150
S-125
S-85
750
850
650
SS150
Suround 55