Well, reading this thread, Nik has a point to some extend.
I feel, i own(ed) both headphones, that the hd650 lacks a bit of the airyness of the hd600. The hd650 is better in alot of respects though.
The better the source, the better the hd650 gets. Also a good amp and matching interlinks are very important. I tried a new interlink last week and all of a sudden the hd650 sounded more neutral, faster and musical to me.
It is just important to match. As it is important with any system, beying headhpones or speakers.
I come from an expensive maxed out electrostatic system. I began to miss the impact and fullbodied sound. The electrostats sound often very transparent and detailed but lack most body, speed and musicallity.
Bass is always a problem for electrostats.
That's why i am building a new rig based on a dynamic headphone. I am happy to sacrifice a little detail for musicality. This is, in my opinion, also the case with Nik. A too revealing headphone, as the qualia is, i think, can also cause fatigue. Al you do is try to listen to the details you can determine and forget about the musicality and fun factor music originally should be.
This is just my 2 cents, but i am also an experienced headphone addict. I didn't own or try as much as Nik has done, but i can fairly say i heard quite a few.