stereo upgrade question - bang for the buck!

[23109VC]

My 2000 has the stock CDR-220. one of the knobs is broken so it's a PITA to change songs on the CD change...I have to use the bu ttons to select a track..not hte knob.. as it randomly skips around if i use the knob..otherwise it works okay.. although I'm not ipressed at all with the sound system.

here is what my car HAS:

stock head unit
stock amp in front trunk
door pods OEM with speakers
speaker in dash
rear speakers - prior owner instaleld the porwsche kit that put speakers in teh rear cubbies... i don't knwo what kind of speakers they are..they are small.. two speakers in each cubbie.

I'd like to change the head unit first. something inexpensive. nothign liek the fancy units I've seen guys here put in. i want something that is stock in size and will be in amber so it matches the stock colors of the other dash stuff / gauges.

i'd like to spend under $500 on everything. if this means doing just a head unit, maybe some speakers, or a cheapy amp or soemthing... i'd be fine with it. i don't want a blaring stereo, i don't want some big subwoofer in my trunk..

i wash onestly wonderign how it woudl sound if I just grabbed a "so so" $250 dollar head unit ... i'd get it installed professionaly by a shop that was worked on boxsters so it goes in right and nothign gets ruined..

do i keep the stock amp? get rid of it?

would an aftermaekt stereo..like the "so so" ones make more power and better sound than the stock unit + amp I have now?

if I were to change ANY of my speakers - which ones would be the berst ones to change?

from my untrained hear - to me, it sounds like my dash/door speakers sound pretty good and my rear speakers are very quiet/underpowered..i have NO idea how it is wired up.. my gut tells me the front speakers are amplified and the rears are not...

but i could be wrong...

thanks.