The best thing you can do to get a tighter low end response (read: set bass on high on the head unit) is to add a small interior amp (probably a four channel). It won't necessarily give you any more ultimate volume, but will dramatically increase the sharpness and linearity of you response. It's the single best thing you can do to improve sound quality. I'd recommend the alpine V12 series amps, as they represent, in my opinion, the best bang for the buck in the industry. I run an old Alpine MRVT357 and it has been a rock solid unit for the past 3-4 years.