I'd go with Ken's advice regarding the mains. Try one, listen, try the other, repeat till you're satisfied.
You should probably set the crossover frequency for the center and surrounds, too. The frequency does NOT have to be the same if your amp allows you to use different ones. It is probably better to set the crossover a bit higher for the center and surrounds, because those typically don't produce frequencies as low and as flat as your main speakers.
I am totally guessing here (since I know nothing about your particular receiver), but what SHOULD happen is that you set all your speakers to small. Then you set the mains to a 90Hz crossover; that will send all the low frequencies below that point to your sub. Set the other crossover at 120Hz - same deal; the center and surrounds will produce frequencies above that point and send the rest to the sub. You may want to use 60Hz and 90Hz or everything at 90Hz or whatever. Experiment and set it how it sounds good to you!
There are very few rules. You do this stuff to make you happy. Well, and to give Ray3 something to write about.
