I don't think 300 ohms is whoppingly high; I believe that is normal for earphones. For whatever reason/s that mp3 player have lower impedance but I don't think that should affect the fidelity (at least that's my guess).
I am interested in getting a pair of noise canceling headphones for my Creative ZEN Micro but it seems like most of them have very high Impedance ratings. I read that you should try to keep the impedance under 64 Ohm's for mp3 players.
The Sennheiser PXC250's that CNET rated very high have a whopping 300 Ohm's! What, if any, are the negative effects of using a pair of headphones with a very high Ohm rating?