I answered you on the HD600 thread - but here it is again .....
You'll get all sorts of answers to this one
From my personal testing when I had the tour unit - I loved both my HD600 and DT880 (250 ohm) with the X5 when I had it (I used high gain). I guess the difference between myself and some others is that when comparing, I carefully matched volumes with an SPL meter and a test tone. I noticed no significant loss in dynamics.
I am purchasing the X5 - should be here in a couple of weeks. I ordered it mostly because of it's versatility with both IEMs and full range cans. It would probably struggle with anything very high impedance or very low sensitivity - but IMO it's great with the HD600. They're not that hard to drive anyway.