I think I like some distortion in my audio. I have tube amps, and I use my Sony TA-ZH1ES over my 120 dB Geshelli amp. I also like my Marantz PM-10 which is very similar to the Nilai thanks to dual NC500OEMs that are bridged.
Only one of two possibilities exists (for me)
1) The high frequency distortion is inaudible when comparing HypeX and other Class AB amps, and it's all sighted bias
2) The HypeX amps sound great because of the high frequency distortion as opposed to "in spite of" the distortion
This graph is common to all of the HypeX products. I have found it more instructive to look at the spectrum since THD+N can be noise or distortion and that composition of that distortion can be insightful.
For simplicity, I am going to measure the power output at 1 kHz and then at the same volume, run the test tones at 1 kHz and 16 kHz. The power output when playing the same test tone at the same volume but at 16 kHz is actually lower. I chose 16 kHz instead of 15 kHz since that's the upper limit of my test tone hearing.
As you increase the power, the 1 kHz test tone is pretty similar but the 16 kHz generates a lot more ultrasonics. But did you look carefully at that last graph?
Let's clean up the noise to see what's going on.
Here, you can see pollution into the audible frequency range. The distortion products are extremely low. Let's keep going up in power.
Not sure how the feedback cleans up those ultrasonics -- that's just what it measured.
And finally some test tones at ~5W
CCIF 19/20 kHz
You can compare that to Sony DSEE processing which adds harmonics...
These harmonics are all "low" from the standpoint of audibility and thresholds. But these measurements are not as clean as Class AB amps which don't have these kinds of ultrasonics. I've never ABX'd my amplifiers since it's going to be hard to allow fast switching with proper volume matching.
However, I wrote this with only the benefit of 1 kHz test tones and real music:
"Switching back and forth, it was pretty clear to me that the Kenwood was a more accurate amplifier
that could bring Emma Stone into the room. But no matter how much my brain told me that I should want and prefer the absolute truth and transparency in my gear, I still personal have the preference for a larger-than-life sound and euphonic colorations that I get with the tubes or my Marantz gear
-- at least for that specific soundtrack"
"I actually think the Kenwood L-08M beats the Marantz PM-10 for delivering the most narrow phantom center image (specifically for the La La Land Soundtrack) but the PM-10 gives me the chameleon of clean vocals with the tiniest amount of tube warmth with all of the crystalline clarity of classical music
where direct comparisons can be made to reality."
With these new measurements and seeing the tube-level distortion for higher frequencies, I think my opinions have some measured correlation. HypeX designs are NOT a straight wire with gain from 20 Hz to 20 kHz.
I still really like listening to my HypeX products.