To figure out the recommended crossover point, check the speaker specs and look for the lower limit of frequency response at -3dB (for example, the lower -3dB limit on the Polk LSi15's is 30Hz). At very least, set the crossover above that point. Some people recommend going up one octave from that point (in other words, doubling the -3dB point) and setting the crossover there, but I don't necessarily go that high. For your mains, I'd try 40Hz and 60Hz.