When people are talking about car stereo systems – or any stereo systems for that matter – they tend to miss out on the big picture. People focus on one particular aspect of car audio. Usually, it is the car speakers. Most people who are getting expensive car audio systems are young people. They want big, loud, and low. The bigger, the louder, and the lower, the better. Although it is possible to get some really tremendous booming sound with car sub-woofer speakers, just having the car speakers doesn't give you a good car stereo system. If you want really good sound, you can't neglect any aspect of it. The receiver, the speakers, the wiring, and even the car design play some role in it.
I would say that the audio receiver is at least as important as the car speakers. The receiver, you see, is what feeds the signal to the speakers. A good receiver gets you a good clean signal at the right power, impedance, and dynamic levels. This guarantees the sound that you need. The best way to do it this to pick out a combo system incorporating both the speaker and the receivers that you like. Different speakers and receivers have different impedance levels. If the impedance doesn't match between the receiver and speakers, you won't get as high quality sound.
Of course, where you are getting the sound from matters a great deal. Nowadays, the most high fidelity sound comes from MP3 players, iPods, and other similar digital sources. Be aware, however, that not all digital recordings are of the same quality. If you're playing MP3s that you downloaded online, they may have degraded signal quality. This can make them sound lousy no matter how good the rest of your car audio system is.
Finally, we get to the part you've all been waiting for: the car audio speakers. It is not just a matter of getting the right speakers, but getting the right speakers for your particular car. If they don't fit right, they won't sound right. Getting speakers that are too big for the available space is a great way to guarantee lousy sound. Putting them in an area where they will be muffled by your feet, by luggage, or by the car upholstery is another way. Take into account the car design and get speakers that are suitable for it. After all, the car stereo system should work as a whole. Every aspect of the car design should be in tune.