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Set of agreements and disagreements

Not only does the power amp have to contend with the back EMF from the loudspeaker, it has to contend with yet another signal that gets generated independantly from the direct input signal to that amp channel: sound waves from other speakers in the room, which would include the OTHER speaker of a stereo pair, or a subwoofer, or any other active speakers in the room.
completely agree, but we must admit that back-emf on one side, and voltage generated by the loudspeaker acting as a moving-coil mike on the other side, have levels way different: the physical mechanism is the same (a moving coil in a static magnetic field), but the later acts upon the whole coil displacement, on several millimeters, the former acts on very weak displacement, in the few micrometers range Note1.
In order to create these conditions in a test of a power amp, I do not believe it is sufficient to use another power amp as the generator source into the DUT power amp channel.
As for me, I do believe.
My rationale is that with this set-up, you can get any di/dt (in the acceptable range) for any dv/dt coming from the PA under test.
With the signal send to the PA under test, and the signal sent the test PA being non-correlated, after a while, all the combinations of di/dt and dv/dt have been exerted.
Put another way, all the reactances range (with energy storage) is scanned.
I believe that it would be much more realistic to use an actual loudspeaker system, and have it be exposed to the output from yet another speaker, in order to fully expose the DUT power amp to what it experiences in the real world
If you were to assess an amplified speaker, I should completely agree with you.
But a stand-alone PA is to feed an enormous span of possible reactances from thousands of possible brands and types of speaker boxes connected to it.
Doing the test the way you propose would just assess the P.A with your chosen speaker.
By contrast, the test done with a counter-PA scans the possible span of reactances.
Thus, the DUT power amp channel would be exposed to:
the original input signal,
the back EMF from it,
the acoustic output of the other speaker generating a completely different signal feeding into the output of the DUT power amp channel,
AND
the sound wave vibrations from both speakers!
Yes, the PA would be well tested for this speaker. Which is great for an amplified speaker.
A test bench and a load resistor, or even just another power amp, is NOT going to duplicate this more complete scenario.
Here, I disagree. As I wrote above, all the di/dt and dv/dt combinations are exerted this way. So, although it seems odd, the test is more thorough in fact.
Of course, the icing on the cake would be to use some Phi Spectral Multitone test signals for both of the stimuli, using different frequency bands on each of the independant stimuli so as to more clearly see where the distortion is originating from.
I agree quite completely. Using your set of tones as stimulus for the PA under test would exert it not only in its whole frequency range, but also with a combination of tones, which is better a test than a pure sine.
However, I question using a sophisticated set of tones for the test PA.
What we need for it is to generate the full span of di/dt for the full span of dv/dt from the PA under test.
A single tone does the job in this matter.

So, I do agree you make a thorough measurement by feeding the PA under test with your sophisticated set of mixed tonesNote2, but I don't see what could bring your feeding the test PA with such set of tones.

BTW, thanks for the reference. Do some generators use PhiSpectral set of tones preprogrammed? Would be great.


Note1 It would be interesting to calculate analytically those value to assess their ratios.
I'll try to do it this WE or one of those evenings.
However, for similar speakers generating sound and "miking" it, an intuition tells me that it will be somehow less than the squared electric/acoustic energy conversion ratio.
Not even peanuts, but sesam seeds.


Note2 Did you throw an eye on the specifications for ADSL drivers and receivers? Those specs include too a set of mixed tones to assess the IC quality.
The problematics of ADSL, with its hundreds of mixed modulated tones where intermodulation between those has to be assessed below some level, is very similar to the one you explain at the beginning of your paper.
Since money involved in ADSL is thousand times the one involved in high-end audio, you can bet some interesting technological "spin-up" is to mine out from the ADSL guys.

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