The difference in impedance can be (and I'm persistant to say that it is) caused by a different cable.
Also, when you take a look at the graphs comparing K612 and K712/702 you'll see a lot of similarities.
To my ears they sound alike with differences in stage. Details, clarity, fun ... if it's not the same, then it's so close that not worth the difference in money.
This reminds me a discussion over Sennheiser and its HD555 vs. HD595 (or now HD558 vs. HD598). They use the same drivers too, different cushions and design though. I'm sure many of you remembers the 'epic' HD555 mod with removing the rectangle foam from behind the drivers which made them almost-HD595 :)
I'm awaiting for the shots of the drivers comparing the one from K612 and K702/712. I'm curious if I'm really wrong :)
I can take a pictures of K612, but I have no idea how to put them apart. There are no screws.
I know that service documentation http://www.akg.com/media/media/download/9352 sugests that these cans can be dissasembled from behind, but I have no idea how ...
Edited by NimnuL - 2/17/14 at 11:26pm