As someone pointed out, all three of these amplifiers are in fact designed quite differently, and our assumption is that since the specs that we measure are 'great' that can't/shouldn't result in any audible difference.
Yes, I believe that is the case. The design of the necessary hardware/software can be very different, but all must have been engineered based on facts and data, and likely numerous measurements done in different stages of the design and build. Just like different design/build by different countries managed to land on Mars. As often cited, amps, including the newer class D amps are not considered "rocket science" anyway. We shouldn't need to go by ears to tell us if audible difference could be there in a tightly controlled A/B/X, and I know you are not suggesting that at all. Regardless, I would love to see more of such tests (like yours) done by more people and use more audience such as what Harman had done in the past, for statistical reasons if nothing else.
I have no doubt you heard the difference (based on consistency in your score), I am just interested in knowing what the real reasons may be. At the moment I tend to think it has to do mainly with level matching, based on what has been cited by others so far. Someone mentioned D.F. too, that could be if somehow the net output impedance were affected enough by the various connections necessitated for the tests to result in FR differences that your obviously very good and discerning ability might have allowed you to pick up on when others might not have. So if there were differences in levels and D.F. resulted in the test process, it could explain the consistency you experienced in your successful picks.
I would argue that the role of an amplifier designer is to create the equivalent of the 'magic wire' that amplifies and doesn't modify the signal in any way. Our approximation of that measurement is the standard AP set that see. We haven't actually validated that that is true since even the AP doesn't use actual 'music' but a small set of tones. In this case our hypothesis is that if on those tests we cannot measure an appreciable difference (like the comparison of the Benchmark and Purifi) then we have achieved the 'magic wire.'
While I have seen a lot of opinions on how are testing methodology was flawed etc, no one so far has provided any numbers as to what the impacts of those flaws are in terms of numbers.
Again, yes I believe that is the case, and there is no argument from me on your first sentence. I thought Peter Walker made n interesting point about having designed his amp without being guided by any listening test. In my mind, he must be a good real engineer for him to be so confident that he wouldn't need to "tweak" based on listening tests.
The 'subjective' part of this test is in fact the order that I chose and to reiterate I can't make a claim about that other than 'it sounded better to me.' I think you may be missing the point which isn't whether I chose 3,2,1 or 1,2,3 rather it is that I consistently chose one vs. the other, 100% of the time and we haven't identified any 'smoking guns' that would explain a directed vs. random result. To put this again into perspective, I am the 'objectivist' who went into this thinking that the results would be completely random, I am just as puzzled by the results as you are.
But the puzzlement doesn't change the data or the results.
I absolutely did not miss your point, and it was in fact the consistency part that made me believe for sure you heard the difference. I wonder why you would think I missed that point when I thought I was so clear, that there are at least too parts, "audible difference" and "preference" one does not automatically equal the other. Sorry if for some reason I failed to make that clear. Clearly I need to improve on my writing skill.
I can't answer your question other than to say, test it out and see what you get. My only comment was that if you can't test them out yourself and you had to choose one, I would choose the cheapest of the 3. I didn't test the NC502MP and if I had and it had been #1 on my list, I would have suggested to buy that, again if you had no other information and had to choose.
I have been trying, and so far there is no way I could tell a difference in the so called "sound quality" between my cheap Hypex and Purifi amps(and also my class AAB Parasound amp). I am sure I have hearing loss in the high frequencies, but I do consider myself well trained to hear differences that I know for sure other non hifi enthusiast with much better hearing could not. I could even the minor difference between my DACs and we all know many don't believe in audible differences between external DACs and the typical AVR's internal ICs such as the AK4458. All such tests were always sighted except I did do it blind with just one other person once, so I also consider all my tests are not valid, and totally irrelevant except for me only. I will never post anything such as "I just added a Purifi amp to my AVR, or upgraded my whatever and immediately heard more details, wider soundstage etc.", because I would be concern that someone may read it and take it as facts. That's how a lot of hearsay started about how one brand is warm, good for music while another is crisp, impactful and better for movies based on sighted tests. In my opinion, anyone who made such claims (whether it was about hearing a difference for the better or worse subjectively) should state the caveats and test conditions, and that's exactly what you have done. The fact that you have taken the time to do your tests with the stated details, in my opinion, sets a great example for others.
Lastly, I really appreciate your detailed response to my post.