The DragonFly does not currently work with older Xperia products, and I have asked the team to investigate why.
With the new Z2 and T2 it "works" to the extent that we can play any supported sample rate and it sounds good. However, currently Android does not support volume control over USB, and we have not managed to resolve that for Z2 and T2.
The result is that DragonFly plays at some "default" level which is reasonably loud, but depending on the sensitivity of your headphones and your personal preference, that might be too loud or too low for you. For myself, I prefer the easy portability of the DragonFly in most use cases over the control I get of the volume in the FiiO E17 which I also use. Resolving the volume control is in our backlog, awaiting investigation. I cannot comment on whether we will solve it or not, so if you like the DragonFly, you should be aware of this situation.