for gains, its best to start with the amp gains all the way down, turn up the HU about 3/4 of the way up. turn the amp gains up until you start to hear distortion or until its loud enough for you at that level.
crossovers for subs should be set around 80 Hz Low Pass, speakers close to the same as the subs but High Pass. thats a good starting point, you can always adjust depending on your set up.