Depending on headphones, some might not get loud enough without a proper amplifier. Then again, some headphones that get loud enough but have a very varying impedance curve can suffer from an output with an impedance more than 1/8 of the headphone nominal impedance. In short, the too big output impedance and too low headphone impedance start interfering with how much voltage gets to the cans at what frequency, and the bad (too high) output impedance starts acting as a voltage divider just like resistors in series do.
Only magnetoplanar headphones are an exception, as they have a flat impedance curve. Then again they tend to be inefficient so you will need an amp anyway. In short, make sure your amp has an output impedance smaller than one eighth of your headphones nominal impedance, after that any improvement is diminishing results. There are amplifiers with near zero output impedance if you want to be sure you can drive any pair of cans or even custom in ear monitors.
And oh, some older headphones were designed to run on way bigger output impedances but you'll have to deliberately seek for them to run into one, so don't worry about that and just go for near zero, like the NwAvGuy Objective or FiiO E7, E17, E11, E9 and what have you.
Varying output impedances between amps would explain some weird ass theories about amps having different "sounds", when the reactive load of headphones tested will interact differently from amp to amp.
Edited by seepra - 5/16/13 at 9:52am