this is far from a scientific answer to your question, but I have a very good friend with remarkable hearing and a a seriously refined taste for classical music. She routinely used one of the older model iPod's for music at work...with the supplied ear buds - and was satisfied with the results.
UNTIL....she tried my new Senn HD580 headphones on the same device. She was amazed at the compromises her iPod made when playing recorded music, the apparent compression and cut off of higher frequencies and the lack of depth to the music. She played the same files though her computer sound card (a high end card, sorry I don't know which one) into the HD580's and I had a very hard time getting my headphones back ! So it was not the aac files or the original recordings, it was the limitations imposed by the older iPod. And the moral of this long winded story.....take some files you are very familar with and actually listen to them on the iPod, or any other music player, you are considering. They try to pack a lot of compromises into the small amount of playback circuitry in the tiny devices.
