If I summarize properly the root cause issue around 4khz, if means that the amplifier needs to be matched with the speaker impedance.
This is very difficult/impossible to verify with the available marketing information.
Imagine what is the user experience with the Onkyo RZ50 on board amps going to safety mode without and warning.
In the old time, on some amplifiers, there was a vu-meter and a Led indicating an overload.
At list nowadays the amplifier manufacturers could put back the overload Led.
We actually do NOT know the root cause. While we can speculate about what I heard and how easy it might be heard, all of the numbers and measurements are done with a resistive load.
The power is <0.2 watts RMS so we weren’t running into overload either.
All I have been able to “prove” is that the differences are real, measurable, and meet the threshold of audibility.
What I thought I heard, described as “attack” for the piano is the lack of compression of about 1 dB into a resistive load.
The difference when compared to the raw digital file does in fact suggest the better amplifier is the more accurate amplifier.