In the summer of 2010 I had some money in my pocket from the recent deceasing of a relative, and I thought, "My car stereo sounds like crap; I'm going to upgrade it". Keep in mind, please, that I am a musician and a sound engineer; I'm very sensitive to bad sound; I listen to my own music in the car all the time for editing purposes.
I thought the sound in my '95 Previa was hopeless, really, but I gave it a little thought and realized that my main problem was lack of bass. Any attempt to add any bass to the sound would immediately lead to speaker farts. And the sounds was harsh, without being crisp; there was no presence to the music at all.
I went to the car audio store, and of course they sold me a much more expensive subwoofer than I was planning on.
It sounds ... wonderful. And amazingly, it clears up all the sound problems I had. My stereo was already subwoofer capable, and once I added the subwoofer and removed the low frequencies from the regular speakers, the sound opened up in an incredible way. I have lots of bass, sure; I can now be as annoying as anyone driving down city streets. But the high end is 100% better as well; it sounds like the musicians are in the car with me. What a difference.