Hi
I have a setup of 4 channels (5.1), sub and center actualy not connected.
I do not use any downmixing, just fix 12channels.
However the setup is just one.
The file says what it uses, stereo, 4 channel 5.1, 6 channels. If only stereo is used, the other channels(speakers) are idle.
You need a multichannel DAC I use REME fireface UFX, and UFX II), or may be you can use the AV Receiver via HDMI for that.
Plays loud and clear