Basically a line driver is something that boosts your output voltage before it gets to the amp. Most people dont need one, thats why they dont have them. If you have a hu with only 2v or less pre outs you might need one to get your amp enough voltage to put out max power without distortion. I needed one in my old system. My new one doesnt need it. I used an old coustic crossover with the outputs turned up all the way to make my old kenwood kick the shit out of my subs.