Right now I'm on Stock Oreo (8.0), for the G version.
It's never worked, even with the older version from here (709?) either on Marshmallow or Nougat. About a month ago the official Oreo 8.0 update got leaked and it also doesn't work with 709 or 790. 790 reports happily that it is working at 192k or whatever you input it, but dumpsys media.audio_flinger says otherwise (outputs through mixer at 48k)
Also, hi-res audio works in Neutron, even in Nougat custom ROMs (All of them are patched to use the AK4490).
One more thing: The stock ROMs have a switch, to select the lower power DAC that's in the AK4961 codec. I don't know if this would interfere with Poweramp at all, but since custom ROMs are seemingly hardcoded to avoid using the 4961, it shouldn't be the problem, right?
I'd say it's pretty easy to get a new Axon7 on Europe, a friend almost bought one for 250 euros on Spain. And there are surely some used ones around.
Any file that you need, or test, just tell me
Edit: I've downloaded a 192khz test file (since i formatted a while ago) and took a screenie of media.audio_flinger. no idea if this would help at all
Right when I select OpenSL HD as the output, the volume goes down significantly (I have to crank the volume up to almost full scale) so that might be an indication of the AK4961 kicking in or something, can't be sure at all.