Here is a wonderful aphorism I am fond of: "Absence of evidence is not evidence of absence." This can have multiple meanings but the one that rings true for me concerns our age old debate about measurements and the fact that not everyone hears the same things. If you don't hear any difference between two products we can draw multiple conclusions. Among the many are:

1. There are no differences
2. Your system cannot resolve those differences
3. You're not listening to program material that contains musical information where those differences are most pronounced.
4. You're not an experienced listener.

That you hear no difference is not definitive proof it does not exist. It is merely an observation that in this particular set of circumstances you do not register a difference. The same holds true for measurements. If we're only looking for a fixed set of variables (FR,THD,IM,N) the fact we missed a circuit's tendency to ring when stimulated by a specific square wave does not mean it didn't ring. It merely meant we missed it. There's nothing wrong with a good measure of hubris unless it is so strong it blinds us.