HD600 are actually a pretty benign headphone load, I wish that folklore would go away. They are easier to drive properly than my jh13 and LCD2/3 are even easier. I drive them crazy at just over unity gain (balanced) actually about 1.5x differential voltage gain on a 3vrms dac output.
provided there is enough voltage, say 10-12v across both rails (as in 10-12v total from -5-6->+5-6v) they are an easy load; sure you wont have much joy with 5v and 9v would be a bit low too for full dynamics, but its not hard to have an amp with enough juice.
as for the test of driving 2 x hd600 at once... guys, 2 headphones connected at once makes them EASIER to drive, the impedance seen by the amp is halved as they are seen in parallel....and the efficiency is roughly doubled (the system by 6db so x4, but 3db each). Someone really needs to include ohms law in the headphone basics threads.
HE5/6 on the other hand are not so easy, not because of their impedance, but because of their low sensitivity
Edit: i'm leaving the above because its true, but I wanted to add some detail and clear up my wording on one factor, efficiency, everything else stays the same.
The loudness out of each headphone is doubled (+~3db) with the same input, the system (if that was relevant) would be increased by 6db, but as headphone listening is solitary and many things can get in the way of the audio filling the room, thats a bit meaningless.
What I didnt mean was what was 89dB becomes 178dB (many will have assumed, just making sure). I also hadnt considered the different physical factors with this part wrt headphones placement in space rather than speakers. I suspect its the other stuff you are asking the professor about anyway
Edited by qusp - 6/21/12 at 11:04am