Unfortunately that doesn't work for me on Win 10, I don't have the option for speaker-fill.
I can tell you that if you use the webplayer on MS EDGE (only!) it works perfectly. It's the only browser that will playback in 5.1 that I've seen. Unfortunately you can't select a higher bitrate stream with the webplayer, but it's ok. The other option I've found is to use a spotify add-on with Kodi, this actually sounds fantastic but is a bit clunky to use.
Re: Is it possible to use spotify with a 5.1 setup?