[Eccentric]

You may want to put in some paragraphs in your opening text. Not everyone is comfortable reading one giant wall of text.

OK I'll try.
I noticed some other sound settings and some more details.. I think I was previously running kodi 18.3 possibly like the august 31st or I believe a release sometime after September 4th in an x64 build.

Furthermore in the video settings under processing: render method is set to DXVA.
While playing back a video and opening its video settings there I set deinterlacing to : Off, I also set the  video scaling method to either DXVA or lancoz 3 optimised or AUTO.
I'd also disable Reinhardt stuff on the video settings but it always turns itself back on and never wants to save my setting of having it disabled for some reason, bug much?

I'm not sure why but all these settings seem to subtly ever ever so slightly alter sound?

Following that heres the important part in expert audio decoder settings I'd have it set to directsound with wasapi default passthrough and then toggle the maintain original volume on downmix and the upmix stereo sound and switch between default directsound and my AV receiver directsound.

Somewhere in there it will sound as if it was never really upmixing previously the volume will get somewhat louder and much more surround and have a far far far more lifelike feel to it while also feeling far more processed? I don't know how to describe it other than the feeling you get when you toggle the upmixing stereo on for a 5.1 or 7.1 surround system but sort of subtly enhancing the existing upmixing setting... sometimes when enabling upmixing you will for a split fraction of a second hear it go very clear and loud, this is just making it stay like that always.
then I toggle the passthrough supported options on and off for the default WASAPI as somehow disabling truehd passthrough and such everything except AC3 passthrough seems to make it sound subtly better too. but it depends on the build of kodi I think as for how good it sounds and how good it works.. on one random version of kodi I tried it sounded amazingly good and i'm forever trying to figure out what it was I did to get it to sound like that. After you've gotten it working in one of the few weeks old version of kodi 18 I think it will work with the windows 10 app store version of kodi too just mostly by using those settings and toggling the maintain volume button.
 
I don't really get why but if I open up a different media player and open a youtube URL with it and then go into the media players temp directory and copy out the temporary audio file from the youtube video and play it back in kodi it sounds almost disturbingly good like you could listen to anything and everything on it all day and never get tired of hearing it in my opinion.