Well, correctly specified THD for amplifiers (since the 1974 FTC guidelines) is from
250mW to rated power, 20Hz-20KHz.
A single THD at rated power is meaningless and deceptive.
Typical examples of correctly rated gear (headline number service manual):
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Notice the FTC rating on this example below has a slightly higher THD than the full power number as THD starts off higher at low powers and decreases towards maximum.
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Below are typical preamplifier THD ratings:
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Phono stage:
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Line stages:
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Notice the DIN power in the above examples at 1%THD at 1KHz and 4ohms. Nobody ever considered the EU DIN (or the IEC) ratings as anything useful back in the day. The only serious rating was the FTC and all the respectable manufacturers who wanted to sell gear in the US had to adhere to it. Now we have a global marketplace, with little impediment to importing gear directly. Stuff without c-tick, UL listings, etc. It's a nightmare.
Standards have lapsed to the point where specifications are a sad joke, and not to be trusted, and in many cases, blatantly deceptive and designed to paint their products in the best light as opposed to the old way, which was a worst case scenario.
The random examples above are from Denon, Yamaha, Pioneer and Sony and all those pieces are in my collection. They go to show that standards have not only lapsed across the board, but gone backwards a long way. All those pieces above are >20-30 years old...