right now external USB powered DACs (e.g HRT microStreamer) and Plex do not work well on phones like the Sony Xperia Z3 Compact because there is no way to control the volume (it’s fixed at maximum).
Please add the ability to the Plex Android app to use hardware attenuation like the below app:
https://play.google.com/store/apps/details?id=com.extreamsd.usbaudioplayerpro&hl=en