I would now just sum it up and say the following to get the AMD R9 390 working in High Sierra:
- do the hex editor thing in the first post
- install the newest AppleALC.kext, Lilu.kext and WhateverGreen.kext in S/L/E with
kext wizard
- rebuild kext cache with
kext wizard
- reboot
And you should be done.
I don't even have my IGP from the motherboard enabled in the bios. The HDMI cable now just connects directly from the AMD GFX card to the monitor.
Good luck to everyone.
@onkz
your first post is much appreciated mate, thanks