Quite a few older ones including the K240M and K240DF, for example. As for recent models, the 2006 generation of the DT770, 880 and 990 can also be ordered in 600 Ohm versions (standard: 250 Ohm, other alternative: 32 Ohm).
A bunch of 80s models used 600 ohm drivers - many Senns, DT880s, DT990s, the aforementioned AKGs and whatnot.
Originally Posted by JahJahBinks And in terms of wearing comfort, which 600 Ohm headphones would be crowned for that?
Possibly the oldschool DT880M/S, though a HD540 is no slouch either, always provided you have semi-recent earpads on there (looks like the DT880 used to use pleather pads way back when, and the original '540 pads were cloth with a vinyl ring as opposed to the later velour ones).
Originally Posted by JahJahBinks So what headphones (not counting discontinued models) are 600 ohms? So far I have the AKG 240 and the three new Beyer models that can be configured to be 600.
Actually the K240M/DF have just been discontinued recently. Don't think there are any more current models with 600 ohms - even though you can still buy (semi-)antiques like DT100, DT150 and HD250-II, these all have somewhat less impedance.
Originally Posted by iSleipnir I found out yesterday that how hard a headphone is to drive isn't all about resistance. 400 ohm k340's vs. HD650's and the 340's took maybe 2oclock and the 650's took about 11.
Does the sensitivity/efficiency (eg dB/mV) of the headphone explain this?