I have a "competent" DAC - the Chord Qutest. I originally had it connected directly to my Roon Nucleus. When I added the optical gear, it dramatically improved the sound. The difference is not subtle. I can hear your rebuttal already: it's all in my head, it's confirmation bias, etc. Wrong. I've tried other pieces of gear expecting that they would improve the sound quality, and yet I heard no improvement and was disappointed. Other pieces of gear, I immediately heard an improvement. Two other people who posted above heard a clear improvement as well, one of which wrote a review about it: https://www.dagogo.com/audio-blast-sonore-systemoptique-signature-rendu-se/
By the way, anyone can try it out for themselves with no risk -- they offer a 30-day, no questions asked return policy. Then they'll know whose word they can trust.
By the way, anyone can try it out for themselves with no risk -- they offer a 30-day, no questions asked return policy. Then they'll know whose word they can trust.