Set the cross over point in accordance to your mains (as they can go lower than any other speakers you have) I have my cross over set at 60Hz, and all speakers set to small.
You don't need to, nor would you want to set it based on the highest frequency rated speaker (the surround), as you'll be robbing your mains of their duties in those ranges.
Nearly everybody sets their crossover between 60-80Hz. There is a reason for this. 1) At that range you get into the non-directional bass frequencies, so letting your sub handle those areas won't draw attention to itself. and 2) above that number is the upper-bass range, which is both directional, and usually better reproduced by your other speakers. (remember, subwoofers serve a very specific purpose, LFE!)
Don't over think it.
Slap some QS8's up on the walls, and play around with the crossover settings. I'd try 40, 60 and 80 and stick with the one you like the sound of best. No harm will come to your speakers, regardless of what you choose.
