So after some more reading it seems like S/PDIF will only carry encoded Dolby/DTS signals. Maybe the audio manager is picking up on this fact?
Anyway, I plan to connect my graphics card to my receiver via HDMI in order to bypass this limitation. I should have read a bit more before building...
My new motherboard has onboard Realtek 7.1 surround (Realtek ALC892 Audio Codec) which I've connected to my 5.1 receiver with S/PDIF optical.
When I view my playback devices, the "Configure" button is greyed out. I can open Properties of my HD Audio Device and run sound tests for Dolby etc...