Stolen from a head-fi post I just googled
Again, I could be wrong as I'm no expert in electronics and electrical audio shenanigans but the general gist seems to be that in some cases, there are benefits to amps even for low impedance headphones - not in every case but in some when circumstances described below are present.
How do I know if I need an Amp? Will it be suitable for my headphones ?
Generally, low impedance headphones are made for portable use, so an amp may not be required. But some people consider there are sonic benefits, so there's nothing against using one. Just use this to check:
To find out if the amp fits your requirements, I'll suggest to take a look at these key points:
•SPL / mW from your headphone manufacturer + the max impedance if possible(not nominal).
•Power specs from your amp manufacturer at different impedances.
•If possible, a graph of how the headphone impedance varies with the signal frequency. Some headphones can have wild variations here.
Use this formula to calculate the power needed:
Power = Antilog ( (Desired SPL - SPL per mW)/ 10).
Note that a 3dB increase in SPL will cause a 2x increase in Power.
For reference, 85 dB is considered to be the limit where long term exposure can cause hearing
damage.
Use 85dB to calculate the average power needed.
If your amp can supply this power, and the headphone impedance falls within the mum/Max load impedance the amp can drive, then go ahead and enjoy your amp.
Also, note that some headphones have higher impedance spikes than others (impedance variation with frequency, remember) as well and then you have the music itself where when certain dynamic passages will require higher transient peaks (higher voltage) here or there once in awhile requesting for more power. Hence its better not to push amps to their limit ( driving 600 Ohms with a portable amp may work, but it can also overload the amp due to its sensitivity and impedance characteristics).
How headphone impedance (vs frequency) affects sound.
In case of MP3 players, where output power is not specified, you can assume that the provided earbuds are already matched. Set the volume to a comfortable level, and now plug in your new headphones. If the volume sounds similar, your headphones are adequately powered.