Though it might be called a subwoofer it really only acts as a midrange speaker. I mean it's only 4" in diameter. My door midrange speakers are 6.5" and could probably put out more bass than the ones behind the seats.

I do notice the lack of low to mid range sound from behind me but it's definitely not clean bass by any means that can actually give a nice "hit" but it's noticeable that it isn't there. Plus, I'm doing it in a fiberglass box that will be painted to match the color of the car. It'll look and sound much better than just giving more power to the speakers behind the seats.