Well, I can't use nVidia GPUs because they don't support 88.2 or 176.4 kHZ audio and I have a lot of FLAC audio in those formats. I'm a bit of purist - I want bit-perfect out to my Oppo BDP-105.
As for the version of Kodi - I've gone back to it. The latest 6/8 version is pretty much flawless 3D playback on my Intel GPU. If I disable all the audio processing, it does bit-perfect as well (internally @ 32-bit which is fine for 24-bit sources).
I had been messing around with the RPi2 since Libreelec (Kodi) does MVC 3D, but the video quality of the SoC is terrible: way too much color-banding/solarization.
So... for the time begin, it's back to the Kodi 17 WIP. We'll keep monitoring JRiver. I did after all just purchase a license for v22 - I don't mind providing funding.
I may even use JRiver exclusively for Audio and Kodi for Video where I think their respective strengths lie. Once you get the hang of JRiver tagging, it's pretty darn powerful. It is however quite a non-intuitive interface! And JRemote needs some love on iOS a well, but a great combo all the same.
Gotta have my 3D though.
Going to keep messing around with various settings and see if anything helps...