"I use the HD 800 through a Grace m902b for when I'm mixing on headphones.
I know of several other classical engineers who have gone down the same headphones route (including a composer who also records).
I find them neutral and revealing - when I first heard them I heard things in some of my recordings (which I was using to test them) that I had never heard before (luckily for me it was just more detail and I had not made any mistakes in the editing).
The HD 800 are certainly vastly superior to the HD 600 and HD 650 (and I have both of these as well - but now exclusively use the 800).
Circle Sound Services"
Here's a video of John in action further explaining their incredibleness