My bets on the AR.
The heart of any tube push pull is it’s output transformer.
And history shows Audio Research have always had only the very best output transformers in their amps.
Some of the Prima Luna models have had sometimes ringing a little in the ultra sonic frequencies, (sharp rise in FR above 20khz) which is usually the transformer..
From Stereophile on that PL transformer ringing
But with the tap matched to the load, this peak was associated with a significant amount of overshoot on a 1kHz squarewave (fig.2), though a 10kHz squarewave revealed that the consequent ringing was critically damped (fig.3), the amplifier maintaining its stability.
Cheers George