Hello fingers
Since bass is more or less omnidirectional to humans, adding a subwoofer to your sound system will not in any way improve imaging. It will only extend the low end frequency response. In regards to low end response, I agree completely with audioguru about the volume of your speaker box. For the best imaging, most of the sound from directional frequencies (read: upper low end, mids, and highs) should be coming from the front speakers, with the rear speakers set for fill. Usually there is some compromise on imaging for the sake of higher volume(in the rear).
1. Set your bass and treble settings on the head unit to the middle setting.
2. Turn any bass boost settings on your amp to the OFF position.
3. Start out with the amp gain to the lowest setting.
4. Start out with your crossover set to 100Hz.
5. Set your balance and fade to whatever you had it on before, presumably in the center (this might be different if you had a sub pre-out or more amps hooked up)
6. Turn the volume up as loud as you will ever listen to it or until it distorts, which ever comes first. If it distorts first, then back off the volume a little until it does not distort. Distortion is very bad for everything.
7. Adjust the gain on the amplifier so the bass just fills in where rest drops off.(At this point you may have a dead spot if the rest of your system does not go down to around 100Hz-see next point)
8. Adjust the crossover (probably up if anything) until it sounds best. Just remember, louder doesn't mean better. You don't really want to be able to distinguish where the amp and sub come in.
Now, in regards to the bass boost. Is it a remote activated bass boost, or do you have to do it on the amp? This can help extend the low end. It comes in handy when you are on the highway and the road noise drowns out some of your bass. If it is on the amp you can either turn it on at this point, and adjust until you get the sound you want or just leave it off.
I hope this helps you out. Most of what I have learned came from a book called The Car Stereo Cookbook by Mark Rumreich. If you intend on doing any further upgrades I highly recomend reading it.
Edit: I almost forgot. You should Make these adjustments with whatever music you would typically listen to, preferably something with a wide range of sounds. If you find that you must make adjustments, then make small changes to the crossover frequency or the bass boost. Also as speakers "break in" their sound may change a little, so after maybe a month or so (assuming your speakers are brand new) You may have to adjust a little.
Good luck