Have you tried ABXing that?
I know I couldn't hear the difference on my portable.
Or even worse heard distortion in the mp3s and not in the aiff files of the same number. Things like that.
The mp3's sound louder than the aiff files like they had been given a certain boost and had noise on the background wich are gone when checking the aiff files.
Not louder like louder in just turning up the volume but a boost in some spectrums of the song.