Hello knock1, was in the same camp as you and thought coax was superior because folks have repeated that over and over. Had been using that glass toslink from the RME ADI-2 to a plasma TV. The cable is pretty old. I don't even recall the year and manufacturer. There are lots of substitutes these days and for a lot less.
Had bought a very good Mogami coax last year just for use with the WiiM Pro. You can imagine my surprise that such a simple cable switch produced such a dramatic improvement. One reason may be I'm in an area with a lot of competing WIFI so the RFI interference is very high. That's my theory anyway.