[codeninja]

Back when I was shopping around speakers, I was often told that 4ohm speakers are harder to drive then typical 8ohm speakers as they need more current.
 
In the headphone land, I've heard that headphones with high impedance such as 300ohm is harder to drive for small devices.  What am I missing regarding this seemingly the opposite cases?

 

[lbj]

High impedance headphones require bigger voltage swings that portable devices often cannot produce.  Another way to think of this is:  say a hypothetical headphone amp can deliver 500 milliwatts into 30 ohms.  It will likely only be able to deliver 50mw into 300 ohms.