Maybe the problem the Nuprime STA-9 with the KEF LS 50: low sensitivity + low impedance at HF.
https://www.stereophile.com/content/bw-dm302-loudspeaker-measurements
(https://www.stereophile.com/images/archivesart/bw302FIG1.jpg)
I measured a B-weighted figure of 90.1dB/W/m—this mini will play quite loud with only a few amplifier watts. However, its impedance plot (fig.1) reveals that it drops below 4 ohms for much of the midrange, coupled with a moderately high phase angle in the upper bass. Wimpy amplifiers need not apply for the job of driving this speaker.
https://www.stereophile.com/content/kef-ls50-anniversary-model-loudspeaker-measurements
(https://www.stereophile.com/images/1212KEF50fig1.jpg)
My estimate of the KEF's voltage sensitivity was 84.5dB(B)/2.83V/m, which is within experimental error of the specified 85dB. This is a little lower than average but 2dB higher than the LS3/5A's.
Somewhat optimistically specified at 8 ohms, the LS50's impedance (fig.1, solid trace) drops to 4 ohms at 200Hz and to 5.4 ohms at the top of the audioband. The electrical phase angle is generally mild, but the combination of 5.3 ohms and –41° at 135Hz, a frequency where music often has high energy, will make the speaker work at its best with a good, 4 ohm–rated amplifier.
More class-D amps have an awful frequency graph like:
(https://audio.com.pl/images/7/3/6/47736-wzmacniacz-nuforce-dda120-audiocompl-lab1.jpg)