the crossover (freq) should be around 80hz, and
set to Low Pass (LP)
the gain should be set to match the preouts
on your head unit.
theres many ways to set it right..
easiest way is to play a song with heavy bass,
the bass/treble set to +-0 on the HU.. loudness off
turn the HU up 3/4 of the way, then turn the gain
on the amp up slowly until you hear distortion..
then turn it back down little bit, so
you hear no distortion..
this is not the best way to set gains,
but the other ways require equipment
most people dont own.
most subs get blown because of improperly set
so if youre worried about it, have a professional do it for you.