I think I may have some relevant experience here.
I know home audio well, and I know what accurate, high-fidelity sound using live unamplified orchestral music as a reference sounds like.
You aren't going to get this in a car.
However, for the 99.9% of audiophiles who don't achieve this at their home and require this to be pleased, there is hope for you.
I have an 01' VW New Passat that I bought used. It had the "upgraded" Monsoon stereo, which sounded like a broken clock radio. Absolutely horrible, and I just didn't turn the radio on for 2 years.
I then decided it was time to investigate car audio. I first went to a small car-audio shop that also sold vintage home audio gear. You know, KLH, old Bose stuff, vintage turntables.
I thought this gave them some credibility, they quoted me $1200 to put in a system that we agreed would:
* have a relatively flat freq response
* have high dynamic capabilities
* be +/- 3 db to 30hz
* have lifelike, and high-end tone and timbre
The owner of this shop enthusiastically convinced me I was going to LOVe this system, and he said he would be surprised if it didn't compete with my Dynaudio/Plinius home system.
Well, I got the car back, and it sounded like it did before I brought it in, except now with the 30-60hz area up about 30db. Hideously awful. Apparently, they put in some cheap Jensen speakers, some crappy Clarion amp, a JVC head unit, and an Infinity Basslink sub.
Needless to say I went back there, and played some classical music for the owner. He agreed it sounded awful, and asked for a second chance.
I gave them one, they messed around with it some more, I got it back, and it sucked just as much.
Thankfully, I was able to recover most of my money. I kept the basslink sub, because it actually wasn't too bad.
So I then did my own research, bought some parts online, and had a friend do the install.
Alpine 9856 head unit - $200
JL XR-650 - $250 (only front speakers, rears are useless)
JL 300/4 amp - $350
So, for about a grand with the basslink, I put this system together.
It is now just about broken in, and is sounding pretty good.
My goal was to be able to put in a CD or plug in my Ipod, and be able to listen to the album without it hurting my ears, or having a major, distracting problem.
I am happy to say that this system easily meets these goals, and is quite fun to listen to. It isn't transparent like a Quad or dynamic like a Wilson, but it sure makes my commute to a work a LOT more enjoyable.
All this for the price of a "middle end" long RCA cable.
I'm sure it could get a lot better than I've got it, and I am interested in persuing that, but if you want better sound in the car, go for it. It is not a lost cause, though you do have to do you homework and your research, and be very careful about qualifying your installers.
PS - If there is anyone reading this thread that knows a lot more about car audio, send me an email! Especially if you are near Boston. As I stated, I would love to take this to the "next level" if possible and not too cost prohibative, and would love the expert advice and experience.