Recently I have been looking into Multichannel audio and have a similar system, STi5, ultraRendu, Denon AVR and I’m running Roon. This is what I have learned, from the Sonore forum, the Rendu will pass multichannel through USB. But you will need a multichannel DAC of which I have found 2. They are not that expensive, one of them got excellent reviews. This is one of them “https://www.minidsp.com/images/documents/U-DAC8%20User%20Manual.pdfor”. Read my 5.1 posts in the Sonore forum. The downside is you will need to run 6 sets of cables to external in and probably won’t be able to use bass management. I tried to use a USB to HDMI converter from the Rendu no luck there either. The converter needed a driver, even for a Mac. How do you load a driver onto the Rendu? I did try it without the driver and it did not work. What will work is a Roon endpoint with an HDMI out, such as an Intel Nuc, many on this forum use this “5.1 FLAC with Roon to Raspberry Pi” I also looked into an RPi not a solution. In my case I was able to play 5.1 through my MacBook, as a Roon end point, with a USB C to HDMI converter. Of course I have to connect it to my system when I want to listen, a pain, and I’m not using the Rendu. I own over 100 5.1 disks that are DVD-A, SACD and Blu. I have only been able to rip the DVD-A discs to FLAC. The others are more difficult to rip and warrants a thread unto themselves. Bottom line I’ll continue to use my Oppo BDP 83 for 5.1.