The amp under test is this one (as many of you probably already have guessed):
https://www.audiosciencereview.com/...r-amplifier-module-analysis-and-review.27852/
From the review, one can see how high or low is the “SINAD” and also the amp frequency response into 4 and 8 ohm load and into a loudspeaker.
In the ABX test here, a classical music sample was used, with high dynamics, big differences between silent and loud passages and playing both a single instrument (violin) and a big philharmonic orchestra. The silent passage between 16 s and 28 s has average level only -44dBFS. The loud passage between 32 s and 60s has average level of -21dBFS, and it is quite a difference. The highest peak is at -3dBFS. So a kind of music like this checks very well amplifier linearity and dynamic range, the complex character of the philharmonic orchestra reveals possible “melting” of sound by intermodulations. At the peak, amplifier output is driven to 41Vpeak output voltage, this to 6.8 Ohm/200W resistor. Peak power is 247W, equivalent sine power would be 123.6W. The recorded voltage from amplifier output was compared to loopback record of the same music sample. The comparison was made in DeltaWave and shown in post #1. Pkmetrics says -80.2 dBr(rms). The DeltaWave difference file is almost inaudible. I made an analysis in Adobe Audition, where the difference (“delta”) file is amplified 100x (40dB) and shown with the original data. Even if 100x amplified, the level of the delta file is very low compared to original, only in case of several peaks, where the output voltage exceeds some 33Vpeak (23.5Vrms eq., 81W/6.8 Ohm), the amp nonlinearity and proximity to clipping plays its game and the deviation from original and rise of delta signal is visible. Visible, but not audible, the plot below shows the highest “distorted” peak, again 100x amplified, but its width is only 317 microseconds. So as long as we are below some 80W, there is no chance of audible differences. This is my point, keep the frequency response invariant of load and then the SINAD number is almost pointless, of course if kept below a reasonable level better than 60 dB. Rather take care about noise measured over the audio band, to be able to play well the silent passages.