I agree that the main well made players may sound very similar and the main difference is probably the earbuds.
However, even a relatively small difference such as the quality of the headphone jack and the internal connections to it can make a big difference in the sound. I wouldn't be surprised if differences such as these do exist between some well known players.
The only way to tell would be to test different players with a good set of buds/phones.
I invested a moderate amount ($90) bucks 2 years ago in some Shure e2 phones. The sound improvement over standard earbuds in vast, and there exist much better upgrades than this model.
differences in mp3 players
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