This..

Do it..

 

your gf loves my body...... BAMMMMM!

 

I think we need to be carefull in assuming that all things digital are the same.

List of things that can affect sound quality:
1: extra D/A A/D conversion step if you connect analog audio out of mp3 player to deck.
2: quality of mp3 generation. Not all dsp algorithms are equal
3: quality of mp3 playback Not all dsp algorithms are equal
4: if you burn MP3s or wav to CD there are error and error correction issues. some CDRs are just crap and your deck may not be able to fix everything.
5: just like everthing else in the world, DSP algorithms are patentable. Your hardware manufacturer may have made some compromises to save costs
6: DSP is a mathematical algorithm. They are not perfect. DSP chips run these algorithms. They are not perfect. Engineers design these algorithms, We engineers, on the other hand, are perfect, we may not be able to spell, but we are nontheless, perfect! ;-)

I stopped using my Blackberry as an in car source. Sound is crap. Dont knwo if it is BB or the extra D/A A/D step. I am burning wav cds from my downloaded mp3s. Amazing difference. (besided the fact that my stock car deck doesnt play mp3 CDs)

I also see alot if differences in the sourc material. Some songs are just marvelously engineered. One beef I have is with Led Zep early years. Some tracks are just so distorted. I dont think its just the fuzzbox that jimmypage used. Unless I am mistaken.
Also some music may have bass at points where your system has bass response anomalities. Same for mids.