I find S/PDIF coax superior to optical. Have you tried using the coax on the transport? I run coax from both my transport and streamer to my DAC.
Another possibility is your transport is not up to the task. There may not be enough jitter correction. You could try aftermarket footers on the transport, use a damping weight on top, try a better power cable, use power conditioning.
Another possibility is your transport is not up to the task. There may not be enough jitter correction. You could try aftermarket footers on the transport, use a damping weight on top, try a better power cable, use power conditioning.