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and again

Your bench test does not include:
AND
the sound wave vibrations from both speakers!
Because it's another test. I agree that live vibration testing is important, not only to assess the mechanical durability of the equipment (the test procedure of military "Ground-Based"equipement should be used), but also to ensure there is no "microphonics" when vibrating in a static B-field. Test should be conducted on 3 axles. When I see the way some high-end equipent is wired or laid-out on a PCB, with current returns remote from the path their currents come from, there is nothings odd at them being microphonics.
But again, it's another topic. Many other things to say about vibration tolerance, but it's another topic, that we should cover on another thread.
If we look at the signal from the loudspeaker very simplistically, then you might think that feeding a signal in from another power amp would suffice to duplicate what goes on with back-EMF, but unfortunately, the timing and phase relationships are not going to be equivalent, the back EMF WILL stress the amp in a manner that a separate indepedant tone (or multiple tones) will not do, because of the current and voltage relatonships of the back-EMF to the orignal signal coming out of the amp.This current and voltage relatonships of the back-EMF to the orignal signal coming out of the amp will cover a large span of the possible di/dt and dv/dt for any given V in the acceptable range of the PA.
The proposed test will, after a few seconds, cover all this span of di/dt and dv/dt versus V.
Which is its intended aim.
Which is why I don't understand your objection. It would only be pertinent if some values of both di/dt and dv/dt were not to be generated by the testbench.
Which is not the case as long as the tones used in the DUT amp and the test amp are not correlated
BTW, you could also use a white or pink noise signal to feed the test amp. While it would be good to assess PAs after manufacturing (against some max reference level of the IIM distorsion), it would be less useful for design verification since the casual intermodulation components would just appear like noise.
I understand what you (and many) would like to do: create a more completely cntrolled and replicable test situation. Exactly.
Unfortunately, if we are to arrive at the real truth of what is going on in the real world, we can not ignore certain as[pects because they are inconvenient or untidy.
 Be careful, you're handling a double-edged axe. Don't hurt yourself ;-))
In fact, I like this test procedure because it exerts the full span of possible situations a PA can get along in its real-world life (I mean regarding only the IIM distorsion or back-emf tolerance, other real world situations like mechanical vibration need other tests)

Yes, use of any particular loadspeaker would technically be valid only for that loudspeaker.
OK
On the other hand, a very typical 2-way system could be used as a baseline...
 Ouch, you just gave yourself a wound with that damned double-edged axe. Feeling fine, I hope?
...and additional test could be conducted on a "typical" electrostatic loudspeaker.
 Back "esf" (electrostatic force) in electrostatic LPs is unlikely to generate more than a few millivolts... (much more with piezo transducers, but who use them in an high-end environment?).
But to be checked in the lab on real LPs.
That said, the span of di/dt and dv/dt for any V being smaller than with electromangnetic LPs, it will still be covered by the proposed test procedure.
Something approaching the whole truth of the matter, even if it involves a specific loudspeaker system, would be better than ignoring or sweeping under the rug a portion of the truth
 Again, be careful with this axe of yours!

;-)

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