I have no problem with 0.1% or 1%, as long as it is a hard line in the sand for consistency in rated, continuous output power specifications. Just pick a number that is in the realm of high fidelity and stick to it. It must be across the audible bandwidth- no cherry picking for the 1kHz 'test' only. Make it a requirement for advertising the product for sale in the US. And enforce it.
Obviously some amplifier designs clip hard and fast, others are more gradual- have a look at the latest little SMSL DA-6 class D @amirm
just reviewed. 1% (-40dB) would take the power he arrived at into 4R from 30 to 50W and from 13 to 30W into 8R- a huge difference.