@keabler: I went on a bit of a DAC/amp spree a little while back and found the Glacier to be arguably the most ergonomically well-suited to use with a mobile phone of the devices I had (iBasso D6, iBasso D-Zero, Apex Glacier, Leckerton UHA-6S Mk.II, and Meier Audio PCSTEP). I actually like the form factor of the D-Zero best of all because it's so tiny, but since the Glacier was roughly the same size as my Galaxy S3, I think I liked it a touch better overall (gave me more space to rubberband to the phone without being too thick). Personally, I liked the sound of the Meier amp better, but found its ergonomics to be wanting for a mobile device (I did a review in a different thread on the comparison of the two). I haven't directly compared the Meier to the Leckerton, but I generally find them to be in the same tier in terms of audio quality and think the Leck has a shape and size that's more conducive to use with a phone than the Meier. There are more options out there to be sure, but hopefully this will give you something to research ;).
@Raenmor: Anything that gets us even a half-step closer to universal compatibility between Android devices and USB DACs is a win in my book-- thanks for the work, man!! If you end up finding a way to create a service that would allow any s/w player to utilize a USB DAC, it looks like that would be pretty easy to monetize that with all the interest around this. There are a few people already working on it (most notably eXtream, who wrote the USB Audio Recorder Pro app), and whoever gets a functional version to market first stands to make a bit of money, I would imagine. Hope you keep up the work ;)
I would think an app that did nothing more than allow the phone to output a digital signal through the USB port wile using any music app would be very popular. I'd pay for it.