The coil reactance also has quite a bit to do with it. Since the reactance is frequency dependent, you'll see various levels of impedance with various levels of frequency. The impedance of speakers and such is measured at DC with a multi-meter. That's the number we see on headphones, it's a nominal value which isn't static across the audio spectrum. Reactance is calculated:
XL = wL
^ read, X sub L equals ohmega L
XL is reactance in ohms. w (little w looks almost like lower case ohmega) is 2*pi*f, where pi is ~3.14 and f is frequency. L is the value of our inductor in Henries.
The value of reactance you find will be at an angle of +90-degrees. In an inductor, the voltage leads the current.
Ohm's law is the most simplistic way of calculating the output power. To get the true value, you have to test the speaker/headphone at various frequencies over the audible spectrum (only a few since doing them all will surely be done after you die, it takes long). It's a bit difficult to put polar numbers into Ohm's law (numbers can't be added and subtracted in polar form, they must be converted back into rectangular).
So, to make my understanding shorter, the speakers/headphones impedance curve has a lot to do with how easily an amp can drive it. If you have an audio driver which dips into impedance levels close to DC (0Hz), your amp will experience stress. Try driving a paper clip with your amplifier, if the DC protection doesn't kick in, you'll more than likely blow your amp.
Please excuse any typing errors, I hammered this out as fast as I could and I hope I didn't bore the hell out of anyone. There is far more which can be added to my answer.