Im going to guess your head unit probably puts ~2 volts, thats really not much at all and im sure your gain will end up being atleast half way up. I usually set my head unit at a decent volume with amps turned down, adjust the highs untill there as loud as I want, and have the sound quality I want. then i leave that alone and adjust the sub amp to the volume I want, usually it takes a while to get it sounding right, exp with the manual knobs where you have to get in the trunk or under a seat to adjust (usually the differant spots in the car will sound differant)