Reccomendation

I have just purchased a Sony 4X50W head unit for my saxo and when i
try and play anything with a bit of bass it doesnt sound good. The car
has the 2 standard 13cm front speakers and the 2 standard rear
speakers. I know nothing about car audio, what would be a good idea to
improve the sound quality? would you changes the standard speakers?
get a sub and amp? The head unit i have bought has 2 preouts would
this allow me to run a sub/amp and stillkeep all 4 of the speakers
wired up?Sorry for all the questions and thanks for any help you can
give me.