What shouldn't have any compatibility issues is HDMI extractor. That way you're pulling the signal directly from the HDMI audio output of the NAS HDMI. You should be able to pull a 5.1 signal, stereo. If I recall correctly some do pass through.
Any PCM2704 based cheapo USB should pull at least stereo. Probably pick one up for like $10usd on eBay. I've never had any problem with one failing. The more expensive option would be a sound blaster x-FI HD but honestly I don't think it's any better than the cheaper ones.
Edit: pcm2704 may not fit your specification at 192. Maybe this: https://www.amazon.com/Douk-Audio-Converter-Interface-PCM192Khz/dp/B085XPRSGM
Why doesn’t this come with USB?
Because adding USB input would increase the cost (both to Kanto and to you), and Kanto’s market research probably showed them the majority of buyers would be using this in their living room or at a desktop computer (many desktop motherboards have optical out), not at their laptop.
To connect digitally to your computer you will need a USB to S/PDIF converter. This one will work fine:
https://www.amazon.com/LiNKFOR-Coaxial-Headphone-Converter-Optical/dp/B07SR7BYD6
Or you could step up to this one which can handle some additional formats and sample rates (not necessarily for most people).
https://www.amazon.com/Douk-Audio-Converter-Interface-PCM192Khz/dp/B085XPRSGM
I doubt you'll find a standalone cable USB/Toslink, the optical signal needs to be generated somewhere, usb only transmits electrical signal, so you need an adapter to catch the usb signal and converted it into light signals.
A note for Mac users: you can achieve this by using the pre-installed Audio MIDI app.
I bought an affordable external USB-to-Toslink converter off Amazon and connected it to my Macbook. Audio MIDI automatically detected the converter as a “speaker” and now allows me to direct Apple Music streaming audio to it independent of all other sound. It also enables optical digital recording onto the MiniDisc recorder.
The only issue is track changes aren’t detected unless there’s a three seconds or more pause — so no automatic audio track markers on live albums or other releases that use gapless segues (eg, radio shows, or Dua Lipa’s Club Future Nostalgia). So I have to go back after the recording is done and use the T.Mark button.
The cheapest route is an adapter cable. You could get a separate USB DAC or sound card to connect to the amp. No need to go more expensive than a Schiit Modi. USB-to-SPDIF converters also exist - XU208.
That’s an understandable concern, but some computers have pretty good sound off the motherboard. If you’re not satisfied with it, you could try a USB to SPDIF converter - Douk U2 - or try a Topping USB DAC. It doesn’t have to be anything special. There are many good DACs.
