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I think someone alluded to a possible problem with that a few pages back, though: if you start with something distorted in some way (for the sake of argument, let's say 50% THD) and you then add an extra 0.5% of distortion and can't hear the difference, you can't then say that the 0.5% is below the threshold of audibility.
Of course you're talking about less extreme examples than that, but the principle applies: if you are talking about determining absolute transparency, you can't do it using a relative comparison without assuming that you already have transparency in one of the cases. I don't see anything wrong with making such an assumption - but it then prompts the question: if you already know what is transparent and possess it in the form of a 45-year old amplifier available for £100 on eBay and £25 in 'tribute' kit form, why even bother with the test?
There are some leaps of logic in your second paragraph. The assumption we have it in a 45 year old design, and even if we did was it known at the time we did. Certainly the Quad is not unconditionally transparent. The frequency response isn't flat enough. Also the idea we can't get anywhere making relative comparisons without an absolute reference implies knowing we are making progress is impossible. Which isn't the case.
In an absolute state of perfection, an amp would in no way at all alter the waveform. But we know from how they are made, how the devices behave, that some alteration occurs. The question is how much is too much making it audible and how little is enough on one can tell it from the pure original.
So how do we get anywhere. We start with 50% THD for an extreme example. And we build an amp with 10% THD and see if there is a difference. There is so we know we are still above perceptible thresholds somewhere between 10% and 50%. We build an amp at 1% THD. And while there is a difference we can notice that with some music it isn't so clearly different as before when comparing 1% and 10%. So we may be getting close to thresholds. So we build an amp at .1% THD. And with nearly all music no one can hear a difference vs 1%. With some pure test tones there is a perceptible difference. So we build an amp with .01% THD and with no sound we can find does anyone hear a difference vs .1 % THD. So our threshold is somewhere in this range. Yet we never had a pure absolute reference except conceptually.
Of course that all applies to speakers or headphones etc. The series amplifier test does give us something very close to a pure absolute reference in wire vs an active amplifier. The following amplifier would also represent a limit. One can let amplifier B power speakers, and then compare wire feeding B vs amplifier A feeding amplifier B. You can then swap positions of the amplifiers and get some handle on which is closer to wire between the two. Once the two amps are close enough to each other it won't be obvious.
Another approach once our measurements and concepts get things well under control is to add adulteration to signals and see how much it takes to become audible. If we have an amp at -100db THD, and we create a clean signal and a signal with -60 db distortion, the fact the clean signal is not reproduced with absolute perfection isn't a confounding factor should the -60 db signal be perceptibly different. And yet we never had an absolute reference there either.
So we can get in or around real thresholds in these ways.
Now if the 405 mk2 or some other amp is unconditionally transparent vs our hearing thresholds why didn't we quit making new amps? Surely this isn't a serious question. Price, marketing, appearances, convenience, features, a switch from class AB to much more efficient and/or powerful class D amps. Things like that.