[gregorio]

  Quantifying the magnitude is one thing.

 
It is just one thing. If we're talking about a magnitude below audibility, then it's the "one thing" which trumps all other things.
 

  Determining how the human brain would perceive the differences is entirely a different matter, and emitting a subjective assessment out of thin air seems like a very silly thing, indeed.

 
You think that's "very silly indeed"? Try determining how the human brain would perceive inaudible differences! That's a whole new world of "very silly", a level of "very silly" which makes the previous "very silly" look positively sensible! .... Welcome to the world of audiophilia
 

  Overall, it's pretty hard to subscribe to the "no diff in transports" paranoia.

 
However hard you find it, it's worth it in the end, just to attain some semblance of being sensible and avoid being mind bogglingly silly! .... Then maybe you'd find more competent engineers around these parts.
 
G