new GAN amplifier

A lower damping factor will, and especially in the case of the Alexia where one just has to look at the frequency response/impedance curve to see it, often correct peaks (valleys) in the frequency response. It is like a built in equalizer :-)

And .... most people don't know this, so I will not fault you, but high damping factor often results in more distortion in dynamic drivers. So no, high damping factor is not always better. Different yes, but better ... not necessarily so.

 A poor damping factor will raise the designed loudspeaker system Q to beyond the point of optimum damping, and because low frequency impedance is typically modulated by the signal itself distortion will always be lowest when driven by an ideal voltage source. Noting too, if damping factor is close to "1" by definition half the power is also being dissipated in the source impedance. That's definitely not desirable. 

Not surprisingly perhaps, 99.9% of high-end commercial speakers are not particularly amplifier friendly and are clearly designed to be driven from a very low impedance source (ie. ideal voltage source). 

arty_vandelay
You wrote:
"Distortion (IMD) is still higher than a decent classic blameless AB design above 10kHz."
Do you mean this part of Amirm's test:
"I was surprised to see the rise in distortion with frequency. I had hoped that the super high gain-bandwidth of the 1ET400A would do away with this. Not an audible concern though as the distortion products here are all in ultrasonic range. [...]The Benchmark AHB2 does a lot better since it has much cleaner ultrasonic spectrum.".
Well, ASR is known for measuring audio phenomena that no human can hear. A concern for bats, perhaps. As for the 18khz+19kHz IMD, Purifi is absolutely tops, not to say that no other manufacturer provides those data. Amirm writes:
"Bruno had encouraged me to run intermodulation test with 19 and 20 kHz tones. I matched levels with Benchmark AHB2. I was surprised to see so much lower intermodulation distortion with 1ET400A. The first pair of sidebands is more than 10 dB cleaner and the rest almost don’t exist!".
The Benchmark AHB2 is Amirm’s/ASR reference A/B amp. "The cleanest amp on the planet".
As for subjective perceptions of audio, to paraphrase the local classic, "there are many, me including, who like tubes best".

As for subjective perceptions of audio, to paraphrase the local classic, "there are many, me including, who like tubes best".

Horses for courses. I don't care too much for tubes but I'm a fanatic for pristine treble, and that includes a ruler flat response out to >100kHz.   

Measurements of the purify look good but keep in mind those IMD measurements conducted at relatively low power and with a LPF in line. 

Btw, the new Halcro Eclipse is now the new "benchmark" for low distortion.   

>100kHz response?  Some species of bats indeed can detect 200kHz but they can't hear the human range sounds.
Then there is the question of transducers, I think I came across once a lab specimen that goes high ultrasonic. Then there is the problem of mics, top studio Telefunken U47 (9k $) goes only up to 20kHz.  So, other than Batman music recorded with lab mics and played by lab transducers it might be a tall horse to climb.

>100kHz response?  Some species of bats indeed can detect 200kHz but they can't hear the human range sounds.
Then there is the question of transducers, I think I came across once a lab specimen that goes high ultrasonic. Then there is the problem of mics, top studio Telefunken U47 (9k $) goes only up to 20kHz.  So, other than Batman music recorded with lab mics and played by lab transducers it might be a tall horse to climb.

Of course we can't hear 100kHz. The desirability of a flat response out to >100kHz has to do with minimizing the side effects from the cascading of low pass filters starting all the way from the recording process itself - as you correctly identify includes tranducers too.  

Always need to be mindful of the time domain.