Excerpted from:
http://www.stereophile.com/content/grado-sr60i-headphones-exploding-myth-about-headphones
Exploding A Myth About Headphones
Headphones are just little loudspeakers—and everyone knows that a loudspeaker with a higher impedance (Z) means an easier load for the amplifier. Why, then, do people always assume that lower-impedance headphones are easier for portable players to drive? Because they're wrong, that's why.
With headphones as with speakers, a lower impedance means a greater demand for current from, and thus more stress on, the amplifier. In combination with high output impedance, a low headphone impedance can lead to alterations in the 'phones' frequency response. That's why, in the measurements accompanying my September 2009 review of the Westone 3 in-ear 'phones, John Atkinson wrote: "with headphone amplifiers having a high output impedance, the Westone 3 will sound forward in the upper midrange." Normally this isn't a problem, because few headphone impedances drop much below 30 ohms, which is usually high enough to avoid serious problems.
So what is the source of the myth that high-impedance 'phones are harder for portables to drive? It probably has to do with the volume level attainable. Most amplifiers are designed to deliver a certain voltage at a certain volume-control setting—but volume depends on power as well as voltage. Ohm's Law says that power is inversely proportional to load resistance. All else being equal, a higher resistance means a lower volume at a given setting. If the impedance is too high, you may not get the volume you want.
But a headphone's sensitivity depends as well on things other than impedance. The Grado SR60is have a nominal impedance of 32 ohms; my Sennheiser HD650 'phones have a nominal impedance of 300 ohms. Conventional wisdom would have it that the lower-impedance Grados would be the better choice for a low-power portable device. Yet, at a given setting of the volume control, the Grados and Sennheisers deliver very similar volume levels. So which is the better match for a low-power portable device? Both have adequately high impedance, and both sound fine through my iPhone—but the Sennheiser, with its higher impedance, is theoretically the better choice, both because it draws less current from the battery-powered player and because variations in its frequency response will be less severe.—Jim Austin