boxed up the presonus, ordered the gen 3 focusrite 8i6... Hopefully the S/PDIF issues are not caused by the fm3 itself. This will be the 3rd audio device I've tried it on...
I have a Focusrite 8i6 Gen 3. Using its SPDIF input from the FM3 with no problems. Cable is some no-name-low-end-claims-to-be digital audio cable, somewhere between 6 and 10 feet. This on Windows 10.