I am currently using the Momentum, and (since I started being interested in hi fi like 2 years ago) it is the best headphone I ever heard. However there are some thing I don't like. On some recordings bass seems to drown out the other sounds. Also, I read that it doesn't suffer from 'sibilance', but mine does. On for example I Am The Walrus the s sound is very harsh.
So I wonder, would the HD 600 prove considerable improvement over what I use now. I use a Meridian Explorer and an AudioQuest Forest USb to connect the pair to my PC. I listen to flac.