Can you borrow/buy-to-try a Mutec MC-3+USB and connect the optical from the cubox to it and go from the Mutec to Phantom optical? The re-clocking in the Mutec should do wonders to the cubox output and your 'happiness' may return... 
EDIT: and if your mini pc has an USB output the Mutec could be connected directly..

EDIT: and if your mini pc has an USB output the Mutec could be connected directly..
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Devialetless!
Roon or LMS/brutefir/DietPi_x86_64
piCorePlayer/ISORegen/HeadStreamer/Yamaha HP-1
250 Pro CI, MicroRendu, Mutec MC-3+USB
Devialetless!
Roon or LMS/brutefir/DietPi_x86_64
piCorePlayer/ISORegen/HeadStreamer/Yamaha HP-1
250 Pro CI, MicroRendu, Mutec MC-3+USB