So I picked up a Pixel XL and boy, has it messed up my DAC setup.
Jump to the Pixel and Nougat. Hiby no longer recognizes the mDSD or the mDAC. There is no popup screen. ES File Explorer still shows a USB device connected, but there's no familiar Hiby connection popup. The mDSD still works with Android USB System Audio, but that means no native DSD or DoP playback. The mDAC, surprisingly, doesn't even interface with Android USB System Audio. Go figure. Both still work with UAPP however, with the traditional UAPP popup, upon connection.
I hope Hiby fixes their USB DAC connection notifier for Nougat, otherwise, I'll have to deal with UAPP's DSD issues on certain tracks with my mDSD.
EDIT: Just A/B'ed the Pixel XL 3.5mm jack vs the mDSD. No contest, mDSD destroys whatever internal DAC is in the Pixel XL.
You may need to select "Audio Source" in the "Select USB Configuration" section of the developer options to get system USB audio to work. I haven't found the Pixel XL headphone out to be that bad when playing files stored internally. Streaming goes very wrong though. A software fix is seriously needed. Strangely, streaming audio sounds fine when output from USB.