1kHz at 5W and 4 ohms load is NOT an "industry standard". It never has been.
The closest we got to an "industry standard" test was 1W (2.83V) into 8 Ohms and that was for running FR plots, filter analysis etc. Good manufacturers will also specify the FR deviation at full rated power.
Example of an Accuphase:
View attachment 375228
Amplifiers are properly rated from 250mW to rated power, across a specified bandwidth (usually 20Hz-20kHz) with a distortion figure which is not exceeded anywhere (that means at any frequency or power level within the specified limits), usually both channels driven into both 4 and 8 ohms.
Example of current model McIntosh:
View attachment 375227
Do you see ANY mention of 1kHz/5W/4R? Anywhere? Nope.
1KHz has always been a cop out for poorly designed products to make their specs look better than they really are, since the dawn of high fidelity. Nothing has changed, it still is.