check the frequency response spec of your subs to see what freq. it reproduces. I would set the amp to allow all frequencies somewhere under 250-150 Hz. I like having my subs do 150Hz and under. That setting also depends on how well your speakers can do at lower freq. Best would be to have the speakers reproduce everything above a freq. and the subs do everything below
EX: Subs reproduce 20-150 Hz Speakers reprodeuce 150-20,000 Hz Like I said it depends on all the above stuff