I could only use my MKVI+ balanced. The SE output on mine had the hum issue all of the other ones did. It was very prevalent in high sensitivity headphones, but dead quiet on balanced operation.
When it came to the Hifiman HE-6, the only headphone amp I heard it on that could compare to speaker amps was the Liquid Glass. After hearing them on speakers, headphone amps always seemed weak.
I mentioned to emotiva a few times that they should make something like that. Their response was usually something like we have a great headphone amplifier built into our DAC! Finally after the mini-x picked up more steam, I posted a thread over on their forums and their top guy said they would look into it. I'm always impressed with their gear (just bought their DC-1 DAC, which I'm really impressed by), so I'll be interested to see what they can put out!
Haha, the XDA-2 doesn't exactly have the most powerful headphone amp, nor is it desk friendly.
I'm glad they're looking into it, Emotiva could really do some good for the headphone amp market.