I'm not Ecksdee, but I am running an Axon 7 as well. Lineage OS 14.1 is a pretty common ROM for Axon 7 users.
Some people use AEX now since that is one of the few Oreo ROMs for the Axon 7, LOS 15.1 hasn't arrived yet but is rumored to be released soon.
I'll report back once LOS 15.1 is available to see if HiRes works on there.
Anyway, my Poweramp Version 709 shows this when I enable HiRes-Output and plug in Headphones:
And here's a screenshot of the OS details, latest nightly, Android 7.1.2, etc.
@maxmp Hope this helps. If you need any more data, maybe a debug log or whatever, I'd be happy to provide that too.