Source number of channels sometimes doesn't work with some devices, so setting it manually to the max number of channels your hardware supports is usually a good thing.
You didn't show a screenshot of your audio path, so I can't be certain what your file is. Google jriver audio path to see how to access it if you don't know.
I can tell you one thing for certain: if your 8805 said it was receiving a 5.1 signal, but was only outputting 2 channel sound, that is a Marantz problem. You might have had a sound field active. Marantz units can apply different sound processing settings based on the type of input signal, even on the same input. In other words, the 8805 could apply different settings to a 5.1 audio stream from MC than it does to a 7.1 or 2.0 stream. It sounds like you might have some of that going on, so you might want to check your settings on the 8805.
You should download a 5.1 or 7.1 test file that lets you ID your speakers, to ensure all that is working properly.
If you are always going to control volume with the Marantz, then you can max device volume, and even disable the MC internal volume. You might want to insert a 5dB cut in parametric eq to allow headroom for intersample peaks and other processing if also playing videos. That will prevent unnecessary clip protection.