Dude, you have no way of comprehensively assessing how an amp sounds for every complex impedance (every speaker) with every type of music. It just doesn't exist in your measurement kit. For what its worth, you would indeed look like a blooming idiot to every amp manufacturer who actually designs/builds amps when you go to him with your dummy load response and claim you now know how it sounds.
Until you figure out the above mentioned (which you won't), i'll say i heard a difference, you can say you didn't hear a difference.
I bet every dac, preamp, whatever else also sounds the same to you eh? What the fk ever sounds different to guys like you? Anything?