I think the clarification needs is "what" do we hear the same and not the same. I suspect we all agree that we are talking about distortions in our reproduction system when we talk about whether we do or do not hear them. This falls in two categories:
1. Linear distortion. If I change the volume, I think most people will hear the change more or less. If I roll down 2 Khz by 5 db, we all hear that too.
2. Non-linear distortion. This gets harder. Best example is lossy compression. Most people are not able to hear distortion at moderate levels simply because we are not trained to hear them. The distortion comes and goes too rapidly and we simply don't evaluate distortion readily that way. Such a distortion though can be learned and once you do, then you will have superior ability to hear it then general population. Linear distortion in small amount falls in the same category. Who can hear a 0.5db drop at some frequency vs others. I have seen people who naturally hear such distortion without training.
I used to be completely blind to #2. I still remember I was shocked when my 128kbps compressed song sounded the same as the CD. Fast forward 6 months later after training and I could not stand listening to the same files!
Similar thing exists in food. There was a time that I hated sushi and if you forced me to eat it, it all tasted the same. Years later and many trips to Japan, and now I can easily tell the difference between good and bad sushi, frozen and not frozen fish, etc. The trick is ability to compare and contrast and eventually becoming tuned to the difference.
I think some elements of this were said earlier
.