Hi all car stereo enthusiasts,
Disclaimer: This is not a flame. I just wish to share something I have learned over the years by being an audio-nut for my home stereo system. And I don't put fancy stereo in my car.
As an 8-year frequenter(sp?) on rec.audio.* newsgroups on usenet, I wish to share something I have learned from reading the posts there from loudspeakers design experts there (you can read posts from Dick Pierce or John Dunlavy--both are speaker system design experts):
Replacing drivers w/more expensive units do not necessarily mean the sound reproduction will be more accurate. Of course, not everybody loves accurate sounding stereo system, and by all means, you should get something you are happy to listen to. The misconception, however, lies in thinking that replacing stock speakers with something that has larger voice coil magnet or higher price tag will automatically result in "more accurate" sound.
The acoustic system consists of the driver, the enclosure that the driver is mounted to, and the listening area. When measuring accuracy, all 3 factors and how they interact with each other must be taken into account. It is also not trivial to measure frequency response and other attributes of audio reproduction accuracy, and most car stereo shops and car stereo enthusiasts do not have the equipment or facility to do so. A microphone, pink noise generator, and spectrum analyzer fall far short of yielding enough information. Optimizing to the information gathered by these instruments sometimes can be misleading and yield worse perceived sound quality.
If you _prefer_ the sound of new speakers instead of the stock ones, definitely go for them, but doing this does not necessarily imply that you are getting more accurate (i.e., flatter frequency response, more linear phase response, better step response, etc.) sound reproduction.
At the end of the day, only the joy of listening to music matters. Modifying stock stereo system can actually produce BOTH better accuracy and more joy of listening, as car manufacturers may cut corners on stock stereo equipment, but sometimes just replacing some equipment with more expensive parts do not yield superior results.
As for shaking every car in 200-ft radius with quad 18" woofers, I do not have an opinion there
Just my $0.02,