I can vouch for the Sony Xperia Z3 series (Z3, Z3 compact and Z3 Tablet Compact) as digital audio sources over USB. They sound very good compared to modern digital transports, especially with the original Sony USB OTG cable (never thought that would make much difference, but it does). For instance, I found them to be almost as good as a state of the art CD player, much better than blu-ray players such as Oppo 103 and 105 from CD, SD, SSD, HDD, better than Oppo 105 + Ethernet, much better than NAS/PC + RPi. Very good CD players still beat them, but not by much, at many times the price... and inconvenience. IMHO the Z3 matches or beats most CD players this side of the exotic ones.
Also, they sound equally good or nearly indistinguishable whether played from internal storage, SD card, or wireless streaming. FLAC vs WAV also sounds almost the same (WAV involves less processing and hence less noise, but a difference is hardly noticeable).
Convenience is unbeaten: they can be placed in the magnetic charging dock and have USB audio out all the time. The Z3 Tablet Compact is the most convenient for that, since the USB out is on low the short side, and the surface is big enough but not too small. They look fabulous in a rig. I am not sure but sound quality seems to be a tiny bit better on the phones vs the tablet, if my ears don't fail me, but they are within 1-2%. For the record, I have Android 6.0.1 on all of them.
Get a (now very cheap) Z3, delete/disable all unnecessary crap, and play back WAV or FLAC from SD cards: I bet you'd be surprised by the quality of this USB audio source. Good enough I'd say.