Some general stuff...

Everything follows the basics of Ohm's law... V = IR and P = IV (or more frequently here **P = V^2/R**)

where V = voltage, I = current, and R = resistance (which for our slap happy calculations we'll initially assume is the same as impedance)

A doubling of power gives roughly +3dB of sound. A doubling of voltage is four times the power, which is +6dB.

A headphone with higher impedance needs more voltage swing to get the same amount of power as one with lower impedance (the bolded formula). On the flipside, they draw much less current. In this sense, high impedance loads are easier for an amp to drive since high current draw is typically what starts causing non-linearities in the signal. The caveat then is whether the amp can swing high enough voltage to provide sufficient volume and headroom (since transients may spike quite a lot higher than nominal listening levels).

That said, some amps/circuits do not like running near their voltage limits as the transistors/tubes may also have irregularities when they swing voltages that are near the levels of their power supply rails.

On to more complex matters is output impedance and damping factor. An amp has an output impedance Z which basically acts like a voltage divider with the headphone load. The headphone sees a fraction of the output voltage according to R/(R+Z). Generally, the lower the Z the better since the fraction will almost always be 1, but there are some cases where higher Z is useful (short circuit protection, opamp stability, etc).

Headphone impedances are also non-linear and vary with frequency, with models like the Senn HD series and Beyer DT series which have rather large impedance spikes at roughly 100Hz which is their resonant frequency. Becaue of this impedance spike, the voltage division actually swings in favour of the headphone and it gains a larger fraction of the voltage at those resonant frequencies, resulting in a volume bump (and most commonly heard as a mid bass bloom)

Damping factor is the ratio of headphone impedance to amp output impedance, expressed as R/Z. The gist of this is how well the amp can control the driver at those resonances, meaning how well can it make the driver do what it is supposed to, instead of letting it vibrate at its resonant frequency. Sonically this comes across as how "clean" the sound is, though is hard to quantify directly. The general rule of thumb is that you want a damping factor of at least 8 for good driver control. This is less an issue for orthos since they do not resonate like a dynamic headphone.

So blah blah blah, in general a high impedance headphone is easier to drive, and if your amp has sufficient voltage swing then you have nothing to worry about.

Edited by Armaegis - 8/11/13 at 12:06pm