The system in my car is a stock unbranded built in '
Infotainment' (what a vile word

) unit that combines a CD player/radio and Sat Nav. It has eight speakers all in all built into the doors (1 driver, 1 tweeter in each) and sounds pretty good for a stock system.
I generally find that I leave the treble boosted to around 40 - 50% to cut through the road noise, and I adjust the bass from about +10% - + 80% according to the CD I'm listening to. It sounds pretty lifeless with the EQ set flat, but a little adjusting does improve the sound significantly. And it's quite loud too, and doesn't distort at higher volumes.
One really neat feature is that it 'remembers' different EQ settings for CD, FM radio and AM radio. For CD the settings are much as described about, with FM stations I have the treble pulled down and the bass generally a little higher, and for AM stations (mainly 5Live and Talksport, which are both 'talk' radio) I have a big treble boost and the bass set flat, or even slightly cut.
My Vectra's engine is a really quiet V6 Turbo so with all the windows up and travelling at urban speeds, it's quite a pleasant listening environment. The biggest distraction at higher speeds in defintely tyre/road noise, but to compensate for this, the player automatically starts increasing the volume of the music (by a user-determinable amount) as the car rises above a certain speed.
I seem to remember that an older version of RealPlayer allowed you to burn CDs and/or mp3 discs that were automatically EQ'd to sound better on car stereos. Does that ring a bell with anyone? I never tried it, either because it was a premium feature (i.e. they charged for it) or because I couldn't be arsed.