I am cool with the head unit though. I have an '04 wrx wagon. I want to get some better speakers to replace the four in the doors. Will I possible be able to get decent bass response out of replaced door speakers only? I don't really need something that can be heard from a block away, but something that can at least output lower frequencies. Any advice is welcome.

The short answer, No.

The weakest part of the system is not the speakers but the head units internal amp. I guess you could keep the head unit and put on those adapters that take the output to preamp lvls and install a new amp and speakers or just a new amp.

If a little extra bass is all you want, maybe you could cut the stock subwoofer wires and install something with a bigger built in amp. Bazooka or something along those lines.

Yeah, if you want to get some "decent base" id recomend a 200w 2 channel amp for the front speakers and if you want more base i know focal has some 5" subs that run off like 80 watts and will fit under your seat. Youll have to have a custome box made however.