I dont' think anyone said amp didn't make a difference in sound, yes there are differences and you probably can tell them apart with relative ease but, amp differences are no where as large as headphones. Amps are one of those things that if a person who didn't know anything about Audio were to test the difference, they would need a prolonged time to be able to tell them apart.
the fact that people have to take the Amps home and listen to them for a feel days to accurately determine the characteristics of each amp already says plenty about how small the differences are. Yes once you hear the difference you feel like its quite big, but lets say if you hand the headphones to your wife or mom would they be able to tell the difference between majority of amps? probably not.