Hello Mischa, and welcome on this board.
with your marantz cd player connected to the AudioFuse ( optical out of your cd player to optical in of your AudioFuse), you'll next have to go in AudioFuse control center and set "Digital in" to "spdif optical"
please note that to fully hear what's coming into your AudioFuse, you have to set the direct/computer wheel on "direct"
also the spdif optical is FULLY supported up to 96khz samplerate
playing music on your Cd player , then you should see the spdif vumuters move in direct monitoring mix panel, downside of the AudioFuse Control Center.
hope it helps,
feel free to come back to me if needed,
best regards,