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If you try this, you'll see what happens...

...at an idle output voltage of 0V.

But you need to know the PA's behavior at any voltage within its specified range.Note

So, I should prefer the following procedure:
The procedure I describe here is to measure the Interface Intermodulation Distorsion (IID).
Our ancestors knew this one, described by R.R.Cordell in
"Open Loop Output Impedance and Interface Intermodulation Distorsion In Audio Power Amplifiers", 64th Convention of the AES, pp#1537, November 1979
.
Nothings new under the sun...


Note: When you make the amplifier work under the 4th quadrant (that is absorbing energy instead of generating it), the more interesting behaviors are to happen at the lowest of highest output voltage.
But, much more important, an amplifier, even well designed, can behave badly if fed a too long time in its 4th quadrant.
For example for class A-B, excessive heating of output transistors in one branch, but not the other one, which would jeopardize bias current.
Or excessive voltage building upon one branch of the power supply in classD amps.
However, those conditions are irrealistic, and never in its music-reproducing job will the PA have to deal with back-EMF when idle more than a few milliseconds.
Which is the main reason i do prefer the old IID method above.

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