>I don't really get all of the assumption that the HD600 are hard to drive.

Because they were ...once upon a time.

The HD 600 was released all the way back in 1997 and at that time, many computers didn't have any built-in audio. Those which did, usually had something that was awful and was barely enough to power off-the-shelf 16/32 ohm, 100+ db/mW headphones. Back in those days, you really needed an external audio card or at least an amp in order to get the HD 600 to a decent volume level without any distortion and ground noise from the spinning hard drives. Audio has come a long way since then but the idea that the HD 600 are "hard to drive, you need a powerful amp" has stuck around and many are unfortunately just repeating it over and over.