In terms of clarity I think both the HD600 and HE-400 are on pretty even. Overall the HD600 are amazing but I do find the imaging and separation to be technically better on the HE-400. The 600 has a slightly smooth, buttery cohesion of the imagery and instrument separation that causes the sound to be very pleasing. In return though, the details are a little more rounded and smoothed out so that details don't come out as clear. Just different flavors imo.
I will say though that the balance of cohesion and clarity on the 600 are great to me. With the HD650, the sound is amazing because those headphones produce even more cohesion that creates an extremely pleasing sound but sometimes... It's just too much butter spilling into my ears. :P
I totally agree with that statement about the imaging and separation.