I just realized that the Beyerdynamic's I was driving were the 250ohm DT770s.
Sorry, I read your post as I was falling asleep & forgot the part about 600ohm drive capability. 600 ohm 'cans' are normally 'desktop amp' territory. Your best bet is to post this question over on Head-Fi.
Once again , sorry as I don't want to give you the wrong info. (like I almost did)