Reviving this thread to chime in with my NAD C658 experience over the last few months. I was coming from a Denafrips Ares II R2R DAC and an Allo USBridge streamer running Volumio. My goals were to reduce box and cable count, move on from Volumio's annoying network issues and bugginess, and improve the sound of a small and challenging room with Dirac.
The C658 did all of that flawlessly. The BluOS platform has been rock solid over wifi, with not a single dropout. Dirac was an absolute game changer in terms of taming excess bass and cleaning up imaging. The built-in phono stage is a nice "extra" that seemed comparable in sound quality to my Pro-ject Phono Box DC, which I then sold. So lots of big wins in terms of usability, convenience, and network solidity.
Sound quality is very good, though my Denafrips Ares II had a more organic and dimensional presentation. The NAD countered with a bit more detail and precision, but I would definitely take the Denafrips on sound quality alone. This is a preference thing - I expect the two DACs are on similar planes of performance overall.
While I preferred the Denafrips on sound quality all else equal, the C658's Dirac function meant all else was decidedly not equal. Dirac's impact on bass performance especially was so positive that it far outweighed any sound quality differences between the DACs.
It looks like NAD's prices have increased, which pushes the C658 closer in price to something like the Lumin D2. From what I've read (not heard), the Lumin probably bests the C658 on sound quality, but the C658's Dirac and Tidal Connect functions mean I wouldn't be willing to make that switch at this point.
I've seen the reports of issues with the sub-outs. I don't use subs in this system so can't comment on that.
Overall, the simplicity, network solidity, and hugely positive impact from Dirac are going to entrench the C658 in my system for a long time, even if the sonic presentation isn't exactly to my preference.
Hope this is helpful to anyone who's considering the NAD.