Ken, try setting Roon to 7.1 and swap rear and surround channels.
I have several Marantz and Denon prepro and receiver products at home. I was using an old i-5 PC HDMI connected to the Denon, where I did need to set Windows sound properties to 7.1 as well. This said, the last time I went to play through this system (a year ago, unfortunately), MCH did not work. Another Roon user reported the same thing. We both put in support requests and Roon never addressed either.
Here’s a link to my support request. Not sure it will offer further help. Failed to initialize Audio device with multichannel DSD (ref#EVAGFB) - #17 by Burkhardt_Petermann JCR