Here´s a tip for you cheapskates, who can´t afford a HP amp, that can deliver the power, that a 600 ohm HP needs.
Use a Tripath chip amplifier instead! You can get one cheap off Ebay, and it has ample power for a headphone. Mine is with the 2024 chipset (6W/8ohm and 10W/4ohm).
You put a 5W / 8,2 ohm resistor in parallel with each of the drivers, so the amplifier thinks it is driving a speaker of 8 ohm, and not a HP of 600 ohm. If you don´t use the resistor, the amplifier will cut off the high frequencies, resulting in a dark sound.
Make sure that the left and right drivers are separated, so they don´t have a common negative (-), like they have in a ordinary jackplug. Positive and negative for left and right channel is separated in a tripath amp, so dont mix them!
This only works on phones with high ohm. On my Grado´s it got way to noisy! (a lot of HIZZZZZ )