Hi there!
Im in the same position as you. So i went to my local dealer and borrowed their head box II for some days of testing. At first i used it with a nuforce Udac ( used as dac only )..The overall clarity got rather much better and overall control increased as well, but the difference was not that noticeable i was hoping for. I went back to the store and got myself a new dac to match the head box. The one i got was the Pro-ject DAC box USB.
The result was even more clairy and low end control, the dac feels very solid and looks amazing stacked with the head box. I don´t got that much to compare this combo with except the nuforce Udac which this combo outperforms by far..
The amp drives my AKG k242 HD and my old sennheiser HD200 with absolutely no problem at all, very good control! again i think the highs are the strong side of this amp, imho they sound great.
So for me the dac/amp combo is a keeper i think, i will give it some more days listening before handing over the cash, but for the money it feels like a good amp. Very good build quality with smooth volume pot and very sturdy outer metal casing.
I hope that i´m giving you at least a tad of help :P