@pma there is another issue here. Again by testing at higher power levels you are straying far away from reality.Thanks for doing the tests, @March Audio . There is a small difference, as mentioned by @restorer-john , though your modules are about 400W rated power, my KA-5010 clone with only 2x25V power supply is rated at 40W/4ohm. Speaking in power terms, talking about 5W makes sense only for the resistive load. With the R//R-C load according to post #1 and constant output voltage of 4.47V, 5W is true for 50Hz frequency, but at 20kHz the power into 0.5ohm is 40W and output current is 9A. This is just at the limit of the small KA-5010 clone. For your amplifiers, this is deep below the limit and to make a comparable test, you should run it at 14.14V, this would be 50W/50Hz/4ohm and 400W/20kHz/0.5ohm. Output current would be 28A/20kHz. Then the tests would be comparable as of the stress created to the amplifiers.
Just one more comment - testing in REW and STEPS is not equal.
REW sends a sine sweep with duration of several seconds for the whole frequency band. STEPS sends triggered sine wave at each frequency, frequency increment in my case is 1/24 octave and at each frequency the wave is of 200ms duration continued by 200ms pause. The whole measurement takes several minutes, not seconds.
Music content at 20kHz is at quite low levels, maybe 50dB lower than at say 1kHz.
So, 14 volts reduced by 50dB is only 44 mV. Let's be generous and say at 20kHz it's only 30dB lower. That equals 0.44 volts which will be only 3.8 watts.
I have already tested at 6 volts which is 72 watts, way more than will ever be required in the real world at 20kHz.
So unfortunately your test has no meaning, its irrelevant.
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