Don't subscribe to this back EMF voodoo notion. Better to lump it to the linear characteristics of the system, like most EE textbooks and applications do.
Sure, you can use either a simple resistive load or the suggested one by OJ or a real driver (can be a cheapy or a crappy one), test FR and distortion and call it a day.
If you don't test it, you won't know for sure and can only assume... is it not ?
How does the output stage react when current is being returned into the amp ?
Do all amps work like a perfect true 0 Ohm 'short' or does it introduce distortions, which ones and how much ?
Has anyone ever tested for this ?
It may not reveal anything at all as the currents are very small.
In case of the HD600/650/800 the back EMF current around 100Hz is half that of the supplied current around 1kHz so substantial, but still they are very small currents.
It is not a textbook test, just an idea to ADD to a test suite and may (or may not) give additional information about the amps performance.
Was just a musing anyway, nobody has to (nor probably will) ever build it.