I am using the DFR with the Sennheiser hd 600 all the time and the pairing is very good in my opinion.
The Audio Technica m50x pairing is outstanding, The soundstage is massively opened, the bass is much more controlled, and the detail is much better.
The Fostex T50rp mk3 pairing is passable, These cans needs a more powerful amp in my opinion.
Ohhh... I have the m50x and want to get the hd600s. This is good news!