This is a very cool topic - thanks for starting it
@restorer-john ! And I look forward to seeing some measurements of older/vintage amps that "could challenge or approach current state of the art amplifiers."
However, so far I don't see many, if any, such amplifiers in this thread. There are many that John has ruled out for a variety of reasons - "just budget," "dreadful on the bench," "stripped down to fool audiophiles," "would be contenders, except for their residual noise," and so on.
By contrast, most of the amps John feels are good so far, he has described as "lovely." Hardly a standard that tells us anything.
So thus far, this is basically an AudioKarma.org-style amplifier-porn thread. If the actual idea here is simply to talk about cool older amps, that's fine. I know I enjoy seeing photos and descriptions of great gear.
But if the intent is truly to identify older amps "that could challenge or approach current state of the art amplifiers," then maybe dispense with the broken-record,
a priori claim that modern Class D amps are "fragile" and start naming some older amps whose available measurements do in fact "challenge or approach" current state of the art in terms of flat frequency response in the audible range: low mechanical and electrical noise; low distortion; high current capacity; ample power reserves; and if it's thought to be desirable or meaningful, good damping factor and/or slew rate.
I don't know what realistic benchmarks should be for all of those figures, but I'd say that audible hum is deal-breaker and SINAD worse than, say -95 or -100 would be a deal-breaker. So too would frequency response deviation of more than, say, 0.2dB from 20-10kHz be unacceptable, and no more than 0.5dB at 20kHz. And I cannot stress enough how little I care about ultrasonic frequency response. Sure, no crazy spikes in the 20-30kHz range is sensible to ensure the amp doesn't stress or damage tweeters, but beyond that we don't need ultrasonic performance as an indirect measure of quality when we have plenty of direct measures of quality we can look at instead. Finally, for power, how about 100wpc continuous into 8 ohms, 20Hz-20kHz - that seems like a decent number to ensure sufficient power in the majority of home environments with the majority of speakers.
My understanding is that quite a lot of excellent older amps can nail the frequency response and provide plenty of sustained wattage over the full audible spectrum. Distortion and noise, by contrast, are generally tougher nuts to crack for older amps. So that would likely be a way to weed out the best from the rest.