Updated my original post with HD8 measurements..
Have to face it, can't be hard on any of them specifically. They are just slightly different voicings of the same phone. HD8 has a small emphasis on the subbass over HD7 and feels a bit more "alive" (hard to explain, trying to interpret the graphs and doing some eq experiments seems fruitless so far).
HD6 just feels overall a bit muffled and bland, nothing really stands out (but not in a good neutral way either). But I can't really say that it's bad.
What I'm impressed on all of them is the ability to really crank volume without any hardness, the highs are on perfect level for that, nothing gets piercing.. and the bass level is perfect (on HD8)!
To sum it up, so far goodness seems to be directly related to model number/price for me.. I'll probably keep them all for a week on my desk. Feel free to ask impressions on specific tracks etc.
Edited by hekeli - 2/25/14 at 1:50pm