Seeing that you don't have a whole lot of mid-bass capability with your components, I'd recommend crossing your subwoofer at 100-120 Hz low pass. Normally you'd want to cross it over at 80 Hz so you don't mess up the imaging of the sound, but, you'll have a hole in your frequency response if you cross it over at 80 Hz. Most people don't actually listen to music and listen for the "image" of the sound. As far as high pass goes, cross it over at whatever you decide to low pass your subwoofers. There's nothing more grating to me than hearing distorted bass coming from rear fill or components. I hope I helped. = )
DEQ-P9 (30band eq L/R, time alignment, 36dB/oct 4 way crossover)
Soundstream Tarantula (for subs)
Precision Power PCX-4125 (for horns and midbasses)
Image Dynamics CD2 comp mini horns
Scan-Speak 7" 18W/5535A midbasses
Subs still pending.