Sadly I can’t give you helpful insights about SPDIF audio passthrough, as I don’t have any optical ports on my machine. My whole audio setup consists currently on plain old stereo via the analog line out to an external amplifier.
You have other options to connect that Logitech 5.1 surround system. Besides the SPDIF input it provides digital coax as well as an analog 5.1 input. Have you tried one of these instead ? Furthermore, is there even an IEC958 output listed in your sound settings ( that should be the SPDIF output) ? Maybe this thread helps, the user was able to get SPDIF output by using the stereo duplex profile instead of pro audio by using alsamixer.