Just picked up this amp this week Here's my initial review. Source is a Pono Player.
Using the Pono in standard mode, the amp works well with my Senn 600's. Ample volume level for my listening is 6-7 on a 1-10 scale (I though Rupert might put an "11" on it just for fun). Also works well with my HiFiMan 400S. Same level.
Using the Pono BALANCED with XLR cables from SurfCables going into the amp, the volume is INADEQUATE in my opinion. I can turn it up to to top and still want more. VERY DISAPPOINTED.
So, I'll stick with single ended AUX out on the Pono and use the 600's.
As for the sound, it is SPECTACULAR, especially with the 600's. By far the most detailed listening I have ever experienced. I am finding that I understand lyrics that I have never been able to before using this system. The 400S's also sound nice, but not as good. I'll keep them for balanced listening from the Pono directly - they provide the volume I like (the 600's do not, even balanced directly out of the Pono).
Summary: The Senn 600's sound great single ended using the AUX connection on the Pono.