I have Audirvana running on a sonicTransporter and only see the option for Stereo. Of course when I elect my multichannel interface there are other issues noted in another thread, but in the macOS version I can select Stereo or Not.
If I use a locally connected multichannel DAC, or we can get the Merging issue resolved, then multichannel is supported on Linux? Neither of those use UPnP (as you know).