I use the function on the Sony systems to improve the high frecuencies (it simulates fairly well that the tweeters are higher up) called Dynamic Soundstage Organizer.
If your speakers are low in the doors, you'll love the Dynamic Soundstage Organizer. It makes the music sound like it's all around you instead of at your ankles. Sony's Dynamic Soundstage Organizer (DSO) uses proprietary circuitry to induce subtle phase shifts, simulating a sound source that is placed higher than the actual speakers in the vehicle. For example, when the DSO function is used in a vehicle equipped with speakers mounted low in the front doors, it creates virtual speakers in the dash to raise the soundstage.
That easily takes care of the high frequencies, but there is just no way to get good base from a set of small speakers. There's no way a 6" speaker will be able to move enough air for some good base; even my 6X9 are too small (considering there is no resonance space on a Jeep) to create any meaningfull base sound. That's why you need a SW with a good box. Now, if you want something small and compact, you can go for a 10" sub in a sealed box. Personally, I went with the 12" SW and ported box, but that's just me. I use the 10" and the sealed box on my daily driver, and it is not bad (I'm actually considering doing a switharoo because of space on the Jeep).
Anyway, if it sounds good enough for you, then great. I personally am very picky about the sound quality (but I have been living ofrom this for over 9 years now, so I guess its understandable), but to each his own. I know I would certainly not invest what my system costs (street value) to put it in a rattle can like a Jeep... I do it because I get it for next to nothing!