This may not be your goal but the cubox’s audio circuits are (like most pc’s, macs and laptops) pretty basic. None of the Cubox range have hi res clocks, nor ossilators. The signal through the optical spdif out is limited to 16/48, and pretty poor; and HDMI 24/96. Optical spdif is probably the least good way of getting a signal to a dac. There might be software solutions in different (non volumio) kernels but they won’t address clocking or jitter very well.
Being as you are already bypassing the audio circuits through USB, if you get an asyncrenous hi-res USB to coax spdif converter, that will take care of the clocking up to 24/192 and your DAC will be fed a much more accurate signal. Leaving volumio/cubox to do the processing, file handling, playing and storage interface, which it does very well. Being stripped down to the audiophile essentials in tems of the rom; and noiseless, vibration free and small form factor.
My cubox and spdif converter are outputting flawlessly up to 24/192 and the sound quality is spectacular.
You can pick pretty good value converters up on eBay from China which volumio will support. Like this!