I was outputting the video at 480i from the OPPO. When I changed that to an HD resolution I was able to get the multichannel DSD from SACD and multichannel PCM from the DVD-A discs. So it is working fine through the DUO.
Although what confuses me is that I never messed with the output resolution, even when connecting it straight to the Denon 3808. So it must have defaulted to an HD resolution on the Denon while defaulting to an SD resolution on the DUO.
I don't know if that is the case but it would explain what happened.
I just know it's working right now so I'm pleased with that outcome. I think this may have happend to me last year and I forgot that the multichannel HBR audio can't pass with the lower resolution.