I have good sound coming from my AUX. It really depends on your MP3 files, if they are lossless quality, 320kpbs, 128kpbs, etc etc. All my music is above 320kbps at least so I never experience any problems with my AUX and my Logic7. Some songs sound fantastic!
Also, it might also be your AUX cable. If you buy a cheap one for a few bucks the sound quality will definitely lack, not as noticeable as your MP3 file quality but still nevertheless present. Get yourself a decent AUX cable and start looking for 320kbps or higher MP3 files.
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