I have information about C-DSP:
The technical director of Audiotec-Fischer replied the following :
“...
the C-DSP does not offer the possibility to switch between different input sources.
The customer has to decide during the setup process with the DSP PC-Tool software which inputs he likes to use, so either the SPDIF input or the lowlevel/highlevel inputs.
It is not possible to use both digital and analog inputs at the same time and switch between these!
If the SPDIF input is defined in the software setup as input , the remote control URC 2A is necessary to control the output volume.
The HELIX Bluetooth-Interface isn't available so far and we have postponed its development.....”

So as you can see, your only solution, would be to use C-DSP with the URC 2A