when you set up your sound gear in your car make sure everyting is flat.
set it to a level you'd norally listen to it at, and make sure teh bass and treble are at the defualt position. Also make sure the eq and DSO(if you have it) is turned off.
Put some music in that you are familiar with.
Then start messing with the balance, fader, treable, bass, sub output.
What you want to happen is have the soundstage infront of you. Like if you were to close your eyes you can imagine the dj or the band playing on your dash board.
You dont want the rear speakers to be to noticeable, or the sub. You want the sub to create the illusion its coming front the top of the dash, the same with the rear speakers.
Once again, its personal taste. My friend likes to hear the sound some from the center of the car, but i find i get fatigued with this arrangement because my ears are tying to correct it subconsciously.
Make sure the music is comfortable at your regular volume and position. Thats very important. Everyoen hears things differently. What sounds awesome to me might sound a bit to toppy or bassy to you. Also an EQ wont fix bad speakers or speaker placement