Where did I mention power? If you're saying an external amp has zero impact on low impedance headphones, you are in the extreme minority on these forums.
There are way too many details to take into consideration for outboard amps to be feasible on-the-go. It is possible to get more performance from an outboard amp, even when you don't need the power, but that's only if the player your using has a bad amp, and if you can cleanly bypass the internal amp, and if the DAC isn't bad, and if both units are properly shielded.. the list continues.
As a general rule, the more equipment you have between the source file and your headphones, the worse off you are. That's why I say outboard amps don't have a place in portable audio unless its absolutely necessary. Though that begs the question, if the headphones are that hard to power, why are you using them portably to begin with?