It will drive HD6x0 without a problem. Mine is pre-set to the lowest gain (1.0/2.5x), not the normal (2.5/6.5x). I just got K340 (400 ohms) in the mail yesterday. It's driving them fine on high gain--what would normally be low gain--between 10 and 1 o'clock on the volume dial. So I have no doubts about its ability to drive HD6x0. Technically, it should drive 600 ohms. Whether that particular combination sounds best is a minor debate amongst O2 owners and I haven't tried it yet.