Yes I own it, and you can read some stuff in this thread I started
https://forum.audiogon.com/discussions/does-this-exist-preamp-streamer-dac-with-mqa-roon-and-more
In general I was not originally pleased with the performance of the C658. But after 200+ hours of burn in, it definitely is sounding better than it started. I used to own the PS Audio DirectStream Jr (recently discontinued) and it sounded far better than the NAD. It's not a proper preamp, however, whereas the NAD is, so it's not an apples to apples comparison. It also costs a lot more. I don't have any experience with Mytek to compare them.
Dirac still hasn't been implemented in the C658 yet, so that may be the killer app that makes it sound a lot better. I wish they would hurry up with the Dirac firmware update. At this point I'm still planning to return mine but I am hoping to give it a fair shake by trying Dirac first. In my particular case, I'd rather up my budget and get more performance. The NAD has a killer feature set, but with Dirac 2 months late already and the sound quality not super impressive, it's been fairly underwhelming. Over on AVSForum there are a few people that have said they are happy with it and a few who were not and already returned it.
As far as which chip it uses, I'm not sure but I believe someone on AVS Forum posted something about that after inquiring with NAD. I don't pay too much attention to that since the chip is only one part (and not even the greatest part, IMO) in dictating the overall sound quality.
https://forum.audiogon.com/discussions/does-this-exist-preamp-streamer-dac-with-mqa-roon-and-more
In general I was not originally pleased with the performance of the C658. But after 200+ hours of burn in, it definitely is sounding better than it started. I used to own the PS Audio DirectStream Jr (recently discontinued) and it sounded far better than the NAD. It's not a proper preamp, however, whereas the NAD is, so it's not an apples to apples comparison. It also costs a lot more. I don't have any experience with Mytek to compare them.
Dirac still hasn't been implemented in the C658 yet, so that may be the killer app that makes it sound a lot better. I wish they would hurry up with the Dirac firmware update. At this point I'm still planning to return mine but I am hoping to give it a fair shake by trying Dirac first. In my particular case, I'd rather up my budget and get more performance. The NAD has a killer feature set, but with Dirac 2 months late already and the sound quality not super impressive, it's been fairly underwhelming. Over on AVSForum there are a few people that have said they are happy with it and a few who were not and already returned it.
As far as which chip it uses, I'm not sure but I believe someone on AVS Forum posted something about that after inquiring with NAD. I don't pay too much attention to that since the chip is only one part (and not even the greatest part, IMO) in dictating the overall sound quality.