You have to go after-market if you want good sound. What does that mean, it means a new head unit, sub, components for the front doors and new 6 1/2 for the back doors. It also means a amp for the door speakers and and amp for a new sub.
There are all kinds of directions you can go and brands you can choose from but in the end if you want better sound, you have to pay for it.
I'm not much of a bass guy but do like quality sound and enjoy my music, so I wanted to upgrade my system and did so with:
Kenwood headunit
10" Image Dynamics sub
Boston Acqustics components for front
Boston Acustics for rear doors
Rockford Fosgate Mono Amp for Sub
Alpine 4 channel for door speakers
I also had a custon box built for more of a factory look!