I've been running the C658 a couple of years now, and it is a wonderful product. Lots of comments on people's experience so far, but was searching for DAC info as well, I stumbled across the forum. The DAC used still remains unanswered so for completeness I'll add it. Although difficult to find, I did discover it uses the ES9028PRO. Source data can be found on NAD site their white paper, and ESS site.
https://nadelectronics.com/wp-content/uploads/2018/10/NAD-C-658-White-Paperwhite.pdf
https://www.esstech.com/wp-content/uploads/2022/09/ES9028PRO-Datasheet-v3.7.pdf
Many other forums have comments that the DAC is an old generation 2 channel, and since the C658 with MDC modules can in fact enable AV features, that seemed odd to me. Hope this is helpful If the wordpress link is not reachable click on the white paper link from the NAD site.
https://nadelectronics.com/product/c-658-bluos-streaming-dac/
Cheers