Mopidy / 5.1 - Channel Audio

Dear all
I have installed mopidy 2.0.0.
As audio-output I use a “TerraTec Aureon5.1MkII” which is connected with SPDIF to my Home-Cinema-System.
This works fine.
Now I have some 5.1 Files (FLAC and DolbyDigital). But the content of these are only sent as stereo-files to the SPDIF.
As far as I understand there are several stages between Mopidy and SPDIF involved such as GStreamer, Alsa.
Can anybody give me a hint how to enable 5.1 output?
Are there any possibilities to check on which stage the downsampling 5.1 -> 2.0 is done?
Any help is welcome.

My audio-configuration is:
mixer = none
output = alsasink

Best regards