[Cyclone]

problem??? no, less ohms means easier to drive. you definately want less ohms for portalbe use because you can get louder easier and therefore it takes less battery life.

 

[Bricolo]

I'm not sure about that
less ohms draws more current, for the same voltage swing

 

[Andrea]

Lesser impedance allows of higher sensitivity, thus reducing the drain from the player (also, most portabe players are optimised for 16 ohm loads nowadays).

 

[Bricolo]

I don't agree on this point.

Sensitivity is often related to dB/volt
So if your impedance is lower, you send more power in your headphones, per volt. (P=U²/R)
dB/watt is much more revealing, but you can't fool people as much with this unit