The gain matches the volume on your head unit. That needs adjusted. Turn the gain on your amp down all the way, set the volume on your stereo at the highest level you would actually listen to it at (usually 75% of max volume on the stereo), go back to the amp and slowly turn your gain up until you hear it distorts then back it off just a little. Now you know the max volume you can turn your stereo up to before your sub starts sounding bad. The bass boost and bass boost frequency turned all the way down. The crossover knob turned all the way up.