The actual standard (which is still in force) for advertised power ratings covers the range from 250mW to rated power across a specified bandwidth with a single distortion metric which is never exceeded at any point across the rated frequency range. It is not a cherry-picked best case THD (at the bottom of the curve) number to plaster all over your products. Or is it one that goes so far up the distortion curve that, just before the unit shuts down, you pick a power number. That, is deceptive. Purifi and Hypex (and some others for sure) just pick inflated headline best case figures because they think most high fidelity aficionados are ignorant. News flash! They aren't.
The rating in place, rules out picking 'inflexion' points, or 'knees' in the curve, because most of the time, the THD+N at 250mW is already way above the bottom of the curve where dubious manufacturers like to quote their numbers. With power amps, I'd draw a line from 250mW horizontally until it hit the upper power curve and you'd be on the money every time. The class Ds clip hard and fast- straight up. Plenty of other amps, are much slower and 'ease' into clipping. i.e. the shape of the curve is way less vertical.
This high powered March amplifier clearly performs very well, and has more than enough power for any domestic situation, but its 1kHz THD+N at 250mW means it would get an approximate rated 0.0025%THD+N from 250mW to 370/380W@4R. That would deteriorate significantly for a 20Hz-20kHz rating. Those numbers are not earth shatteringly excellent or game changing, but they are good.
It is not a 1kW high fidelity amplifier. It is well into complete non-linearity at or near that power output. That is the point Amir was making and it stands.