I haven't got the foggiest about Android audio frameworks or USB standards, but if I connect my new Stoner Acoustic UD110v2 to OTG cable, plug it into my Moto G (KitKat 4.4.2 ; version 174.44.9), I get audio out of the UD110, at a default line out level, for all apps (Spotify, Google Play Music, Youtube etc.).
This level of audio output is too "hot" for my poor old Fiio E5 amp, resulting in audible distortion during playback at the very loudest passages of music, where there is loud low frequencies etc. Ringtones, alarms and notifications all come through the phones speaker. If I launch USBAPP (trial), any mp3's I have on the device will output from the DAC, and I am able to use the apps volume fader to adjust the volume, thus limiting any distortion in the Fiio E5. It sounds really good to my ears. All mp3's are 16 bit/44100. The only issue now is that if I go back to another app after having used USBAPP, the audio from the other app comes out of the phones speaker (as opposed to the DAC). It will require a reboot to get the other apps audio routed through the DAC again.
However, once I reboot the phone with the UD110 still attached, once booted I can control the volume output from the DAC to the Fiio E5 using the phones volume controls (physical hardware buttons & software slider) for Spotify, Play Music, Youtube etc. All the phones audio will be routed through the DAC. I could even dispose of the Fiio E5 and comfortably power my Blox M2C earbuds directly from the UD110's line out. Consensus however dictates this is not advisable, so I route the audio through the Fiio E5, and set the phones volume to 60-70%, thus limiting any distortion. If I set the phones volume to 90-100%, the distortion returns. Once USBAPP is launched it will work as handsomely as before. Once I quit or switch from USBAPP however I get no audio from any other app, or any ringtones etc. Rebooting without UD110 connected will restore to default audio ; rebooting with UD110 connected, on board volume control of all apps is reinstated.
I have verified that the DAC is not producing the distortion by running Spotify audio from the Moto G and my wifes iPhone 5 (iOS 7) out to my Denon Ceol Piccolo through the UD110, to analog line input. In fact the audio sounds sweetest from the iPhone source, but even the Moto G betters the Piccolo's built in Spotify source. For reference, I used Aphex Twins Girl/Boy as my test track (how very scientific eh). I'm fairly happy that I have found a very inexpensive DAC that I can use at home and away, but am still a bit confused/frustrated as to the whys and what nots surrounding Motorola/Android implementation of audio over USB.
A notification for another system update for the Moto G (176.44.1) appeared on my phone today. This update resolves, amongst other things, support for external USB drives connected by OTG (nice). I will await confirmation from Motorola that audio via USB will not be broken by this new update before I will update. Unless anybody else can verify the update is good to go here first (are you listening jellofund).
Edited by swab - 2/26/14 at 3:34pm