Rok, you stated that you heard no difference between amps; neither did your favorite reviewer. The differences are subtle for me, and not worth paying for when you get to very expensive, but I'm sure differences exist for those who can hear them.
A long time ago when I'm sure my hearing was much better than it is now, I participated in a frequency response experiment; it was displayed on a decibel meter. When the frequency got past 16K the apparent volume began to drop off. Past 18K I could barely hear it. At 20K, no matter how loud he played it, all was silent.