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I discovered that using "cheap" optical I get 90% fidelity straight away. With a regular coax I get maybe 70-80% and with a good (expensive) coax I'm now getting around 95%**. YMMV..
The reasons have to do with quality of parts, inherent superiority of coax (more bandwidth) and the limits of the optical converters.
While coax s/pdiff is superior in principal, it is also more prone to cavots because of bad connectors, unmatched impedance of cables and parts and noise issues caused by electrical coupling of source and pre/pro. A good coax cable has (far) less of these issues.