Just a really beginner question (sorry for the long post - just need advice)

I have a stock '97 Contour GL, and just installed a CD player instead of the factory tape unit. Now I can play CDs, but the system still sounds awful (no bass, no high freq sounds, lifeless).
I suppose it's because the cheap stock speakers.
Right ?

Should I:
- replace all 4, with 2 identical pairs
- replace only the front pair
- replace with 2 different pairs
for best result.

The CD player is the cheapest Rockford-Fosgate model (rfx8310), so I think too expensive speakers would be just overkill.

Do you have any recommendations (or price/product range I should think about or avoid) ?
I have some EE background, and zero knowledge of audio systems.

Thanks, and sorry for the long posting

Tivadar