also factory head units are notorious for having bass boosters built in (60 hz). When we replace our factory units with aftermarket ones they dont have them hidden in the background so the bass sounds weaker unless you crank up your xover. your unit has about 2.5v rca output. thats good. a line driver would help you to turn the amp gains down but wont realy increase the bass. have you played with the sub output on your head unit yet/