"The one single thing that can improve the sound of your car stereo the most is the addition of a subwoofer."

I disagree. In fact, I'm almost sure I heard it put this way: "The one single thing that can improve the sound of your car stereo the most is the addition of an amplifier."

IMO, subs are for people who want others, outside the car, to hear their bass. You can get perfectly good, clean bass out of a set of 2- or 3-way speakers, that's plenty loud enough for any human being.

One example, although I know it may not be common, are the Pioneer 6x9's I run in the rear of my car. The low end of the frequency range dips all the way down to 25Hz. And if I remember correctly, the average human can't hear anything below 20Hz.