You can't have a test that proves differences cannot be heard. That is trying to prove a negative.Agreed and plan on doing it when the queue of more important things is gone through.
No argument with any of that.
I have no beef with your logic, but I have not yet seen the conclusive test. Harman's speaker tests are probably not going to convince some "flat earthers" that scientifically designed loudspeakers are preferred by more listeners than magically designed speakers, but I do accept their test results and testing methods. I would like to see this sort of test of these measurably "transparent" electronics before claiming certainty.
To me there is still reasonable doubt.
All you can prove is that the peeople involved with the test can't hear it.
What is requried is a test that proves a difference CAN be heard. That can be done by just one person. So far - nothing conclusive. But don't be surprised (if one comes up) that person is going to be "grilled" because the result is SO unlikely. It is far more likely to be due to faulty test set up (Getting this right is hard), than to actually show a genuine difference.
And we are not talking about speakers - they are clearly differentiable. Amps - the jury is still out for me, especially when close to limits, though differences - if any - are likely to be of little consequence. But Dacs are pretty much a done deal.