In your first post you mentioned the iPhone. If you are going to use an i-device then you are probably better off with 32ohms or 80ohms, in that order. I have the 250 ohm version of the 770Pros and they sound much better with a proper amplifier than just any ol' headphone jack.
In general, the lower the ohms the easier it is to drive. 32 ohms is common for portable use.
Edited by brunk - 3/3/13 at 9:39am