My subjective opinion when I had nc400 was that they were not perfect in the treble region. But damn perfect in bass region. Subjectively.
Maybe this is confirmed now then here with the rising distortion in higher frequencies?
Amir, will you do subjective listening test?
Here's an interesting thread on performance of the amps.
https://www.stereo.net.au/forums/topic/100666-hypex-nc400s-are-they-musical/
I believe the upper register is fine but certainly nowhere as spectacular as the bass, and I wondered if my Denon AVR-X4300H was the limiting factor? I should mention I am using it as a PRE/PRO configured at 5.2.4.
After reading literally every thing I could find, I came to the conclusion the current batch do not provide the level of improvement I was after.
Amir has said the ability to switch off amps can turn recent AVRs in to excellent processors.
Mine doesn't have that option but I discovered turning ECO on can reduce the voltage to the amps by around 30%.
https://www.avsforum.com/threads/denons-eco-mode-what-does-it-do-exactly.2089586/
It helped and there was only a slight improvement, nothing like I hoped for.
Now the interesting part.
I opened up the case of my 4300 and physically disconnected the inputs to the power boards. The improvement was simply amazing and upper frequencies are now as stunning as the bass.
The only conclusion I can draw is that surround processing demands stunningly perfect, stable and clean power supplies and the inclusion of extra features and channels puts additional stresses on more compact power supplies.
I hope other members might be willing to do the same and have the results tested.
Here's images of the connecters I disconnected. Without removing any components, it is a delicate operation as space is tight. Front and Rear has been written on the images to help locate the position of the connectors.
I should mention that disconnecting the power supply connectors does not work as the Denon quickly shuts down.