And it works surprisingly well with the HD650. I know some people call it bright, but to my ears it's more like "wire with gain," which is perfect for a little pocket amplification solution. I've yet to run into any headphones that don't sound great with them.
I was just answering the poster above, haven't tried it since I have no need for portable amps as my X3 and X3ii are more than enough for the headphones I travel with. But if I were to get a portable DAC/amp then it will probably a Hugo/2 if I ever decide I want to take the HD650, HE560, HD800 on my travels, highly unlikely but never say never.