I'm keeping mine
I wanted to share my thoughts and experience with the NAD 658. At this point I think it sounds great with awesome recordings in my setup. Some recordings don't measure up and there is disappointment depending how attached I am to them.
I agree with the comments that the sound can be clinical. Think that is true especially when first listening. The DAC is revealing. It was easy for me to get immersed in the detail which detracts from the emotional aspects of the music. I've spent enough time listening now that detail is not distracting to the presentation. I like the open wide sound stage, clarity, and dynamic range improvements.
I haven't experienced the fatigue, though I think I have heard some music being edgy/shrill (esp. at first). I haven't used it, but the treble control specs +/- 7dB at 20KHz (pretty high). Might be that lowering it could help. The other option would be paying the $100 for full Dirac (when it is available)
I had issues with the bass that I had to work at to correct. The main issue was level setting for 7.1 inputs. I had to redo the HT room correction after adjusting the sub volume. If the sub level setting is too low it will only activate at very loud volumes. I got it matched to the speakers such that level setting was -3dB and L & R speakers were +1 and +2dB. I then duplicated these values in the 7.1 set up.
I run my L & R speakers as Large. I didn't like setting BluOS app configuration to indicate a sub was present. It created a hole. (my sub couldn't fill in what was missing). Likewise having the sub and speakers running full range made too much bass. This is what Dirac should correct, but as we all know it isn't ready yet (very disappointing as was supposed to be ready at end of January). I fortunately have a Velodyne SMS-1 so I was able to mostly fix my bass issues.
The BluOS app is a big improvement over the Oppo app for the BDP-95. To get the full benefit of it I had to modify the Album metadata id to make separate CD quality and HR quality copies, appear as separate albums. It is nice to abandon the primitive folder view to select music. My one complaint is selecting a song to add to queue can be tricky. If screw up you start playing the song immediately or possibly moving to another song already in the queue if you miss. It seems to require a deft, firm tap between the quality indicator and "..." to work consistently.
I haven't tried Tidal. I did buy a few albums with MQA encoding. Playback works well. I was surprised initially to see that the actual playback is 48 or 44.1 / 24 bits regardless of the bit depth of FLAC container file. Did some reading and gather this is normal. I guess the benefit with MQA could lie with streaming or that the studios will release better sources because of the copy protection. There is a fair bit of controversy about MQA on the web.
With the Oppo the best play back was disk or using a USB thumb drive. The C658 does best with the NAS storage. The NAS is available right away while it appears to take some time for the USB drive to mount. From the NAS, it works great, no issues with file size, gapless playback, always on.
Another thing I had to do was to set the Bluetooth to manual. The default auto setting broke the connection between a Amazon fire and Harmony hub.
Took some doing but now I quite pleased with the upgrade to my system!