Hard to say without knowing how Gene set up his test. If we assume he has an 8 ohm load connected, then assuming the gain of the A6A is about 29 dB (I am sure it is a good guess), then based on the manual, the spec says with two channel driven, the A6A would output 165 W into 8 ohms at 0.9% THD at 1 kHz, we can estimate/by calculation that the clipping point won't be much higher than 1.3 V if Yamaha consider 1% THD is a good indicator for clipping.
Based on that, I would say at 1.5 V the A6A's power amp will definitely be well pass its knee point. That's the theory and my educated guess. Now take a look of the measured results:
You can see that at 1% THD, the output into 8 ohm is 186 W, now if we calculate the pre out voltage, it will be abut 1.37 V. That sort of support my earlier estimate that at 1.5 V, the internal amp would have well passed the clipping point.
View attachment 52910
That does not mean that at 2 V the distortion would become so high that it would be easily audible, because hopefully only a minute portion of the distortions/noise from the badly clipped power amp get fed back to the preamp side.