if you want a more rounded sound, replace the speakers in the factory locations with mid drivers, no added tweeters, and get a decent set of components up front
as i have said above, i dont rate 6x9s, i have had them in previous cars and now realise how bad they after my last couple of systems
i personally see no point in having any treble from the rear, yes, the passengers get a listen, but you dont put a stereo in there for them, it is your car, you are the driver and you have to put up with the stereo all the time, set the stereo up to the way you want to hear it, and dont worry about what the passengers say