well, subwoofers are designed for bass, so unless you want a subwoofer, you arent going to be able to get too much low end sound.
The best you can do without bass is a decent set of door speakers, preferably a component set ran off an amplifier.
You will also see better output with properly deadened doors. I have double-layered deadened doors with pioneer premier componenets on 60RMS, and they can play clean down to around 50hz, and fade out around 40hz, which is perdy darn low for speakers. If that doesnt make sense to you, subwoofers commonly play 80hz and lower, and speakers play 80hz and up.