Yamaha B-4 Experience.

In the eighties Yamaha produced three amplifier models (B-4, CA-S1, CA-R11) that featured an ' Ro Control'.
This control allowed adjustment of output impedence from + 1 ohm through 0 through - 1 ohm.
I am presently studying an old factory tutorial manual and this gives the argument that with this circuit the amplifier op and cable impedences and driver resonance can be eliminated and in so doing provide better damping of the load driver.
Does anybody have experience of these amplifiers, and in particular critiques of the usefulness of the 'Ro Control' ?.

Thanks, Eric.
 
This is the tutorial example circuit.
 

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