I own the heed canamp for many years now and the HD 600 is a good match (for my ears).
If i hook up the HD 600 on the Meier Opera i say it sounds fuller, on the heed canamp the sound is a bit faster.
It's a matter of preference, for instance i lisen a lot to the pearl jam live concerts and i like the signature for live concerts on the heed canamp with hd 600.
>For a studio album i prefer the meier opera with HD 650.
Also don't forget the source, a lot of people give there opinion of a combo without considering the source.
For the sennheiser cans i use my marantz cd6000 cd player. It has a neutral/bright sound. The combo with my denon dcd 755 ar is different.