I guess what I don't understand is the following:

• How an an amp with the L-W topology would actually be able to create 2nd harmonic distortion which just happens to be opposite in phase to the distortion of the drivers to cancel the distortion
• Why anyone in their right mind would build an L-W topology with so much distortion at normal listening levels that it be able to cancel a significant part of the distortion from the HP1000 driver (since distortion from drivers is magnitudes higher than any amp.)
• It's a fallacy to think that amps somehow were more primitive or not good in the eighties. The state of audio hasn't progressed much for sixty years. In any case, the HP1000s do sound better out of low Z amps than high Z ones.

Also Joe made the HPA1 to power his headphones. I believe the HPA1 is similar to the RA1 - an op-amp based head-amp with fairly low output impedance.

I don't mean an amp with L-W topology.   I mean how it works.  The first tube has 2H of one phase,  the second tube has 2H but inverts and cancels,  so net reduction in distortion.

The analogy would be replace the second tube with the headphone.   Have your measurements shown lower distortion at your mic from an 2H predominate amp with Grados than from a heavy NFB amp ?   Again R&D and just an interest, cause we know the ear adds some ungodly large amount of 2H.  Just curiosity.