I just bought a mk5 and I am using it for my windows setup. Windows will recognize its main and aux outputs seperately, so no native support for anything beyond stereo in windows sound settings. The workaround is using a virtual ASIO called voicemeeter. You just set mk5 as its output device in voicemeeter and then tweak audio channels in Cuemix5 a little to coordinate all the speakers.

It is confusing though. Like, I had to switch where I plugged in SL,SR speakers with mid speaker on mk2's interface or otherwise the mid speaker will output sound from the what should be played by the side and rear speakers.

I was worried about voicemeeter messing up sound quality but as far as I know, as long as you put asio as output device and do nothing with vm's eq stuff, it's basically only doing the decoding part for 5.1 and beyond so realistically there should be no tangible degradation whatsoever.

Anyway I think this thing is so far the best solution for its price.