that was the same exact question I just asked in another thread. I plan on buying image dynamics ctx65cs components in the front and that's it. I eventually want to get a 10 inch jl audio sub but that is later down the road. The real question here is, are speakers like that or something comperable powerful enough to give you a full sound throughout your car? Now it also differs when you have a four door as opposed to a coupe.
I think what I'm about to get to will REALLY answer your question. People that are extremely into sound quality want to listen to the music and be able to hear each instrument accurately. When this type of person is listening to music, they are picking out each individual instrument and accurately comparing it to the real thing. Obviously no two instruments of a particular type produce the same exact sound, however they are doing this on a more general basis. The listener quickly processes the memory they have of the live instrument. They remember how it feels, they can remember the harshness, the decay time, the mid bass and every other aspect of the sound. Now for the SQ guys that read this and said "wait a minute"
To best describe how you hear music is VERY difficult. It's a feeling. These SQ guys listen to the music, but what I'm saying is that they are able to accurately compare and contrast the reproduction of the music to an expected sound.
I'm going to go out on a limb, not that I already haven't... and say this: In writing this post I answered my own question and I have a feeling that you are like me when you listen to music. I love being surrounded in sound, I love feeling the warmth of the vibrations off of my body. I like the pure pressure of the low end (sub), yet I like a compromise between that and quality. Cars that have two 15 inch subs beating down the street in my opinion is stupid. The bass overwhelms the mids and highs and you are just completely surrounded in a mix of muddy/crappy low end sounds. For some people, that does it. I like feeling that bass thump my chest a bit, however only to the extent of when it is needed. If you have ever stood about 2 feet in front of a bass drum and had someone step on the peddle you know what I mean. Yes there is "muddiness" in a front/rear speaker car setup. This is because of sound waves coming from the front speakers at one angle/distance mixing with the rears at another angle distance and PERSPECTIVE.
Go to a car and take the front/rear balance on the head unit and turn it all the way up to the front. Listen. Then put it back to normal. I am definitely getting 4 speakers. I need to be surrounded. (Some serious sq quality people HATE surround setups. I like them. NIN has an album mastered in surround sound. Buy that, listen to it, and tell me that you don't like being surrounded in sound.