Most HiFi magazines tested 'at the onset of visible waveform clipping' or words to that effect.
That is how I have always tested as well. It is a repeatable and easily demonstrated limiting point and can be accompanied with the visual depiction. Obviously, at the point of waveform clipping Amir's AP is going to go virtually vertical on the THD vs output voltage plot just as I have to quickly switch ranges on an analog meter or watch the harmonic 'spray' on an FFT.
The onset of clipping test isn't hard to do and anyone with a decent scope, a few loads, a signal source and a judicious hand can get very accurate results for power output. That would mean plenty of other members could contribute comparable tests for other headphone amps for comparison purposes.
With analog amplifiers, it is not as black and white like hitting 0dBFS on a DAC where spikes appear and disappear within a fraction of a dB of level change.
I'd like to see the AP's plot of THD vs output voltage/power and a onset of clipping DSO/CRO scope shot with AC RMS scales. Headphone amps are no different to normal amplifiers and I can't see why all the normal traditional tests can't be applied to them also. So there is single and both channels driven (to show PSU regulation effects) and dynamic tests too. All the others are pretty much already covered.