Yeah, you'd be surprised how adding just a better head unit will make everything sound better. In my car I kept stock speakers and add subs along with a new head unit and it sounds quite good. I mounted my bass control knob under the dash, sort of near the steering wheel. Pioneer makes a pretty good CD/MP3 player for around $200.
FYI: 50 W x 4 is really good. Stock head units usually don't push more than 20 W per channel..