I agree with you and luisdent completely - I am not talking about compensating for obvious differences in sound. For example, I have never liked the HD650 and no amount of listening will make me grow accustomed to its signature.
What I am talking about is what many people refer to as "burn-in." It is simply acclimatization.
My first pair of "serious" headphones was the HD 650. I always thought they lacked "life" but I had nothing to compare them to. The first time I heard the HD 800 I thought they were intolerably bright and thin sounding. Eventually, I replaced the HD 650 with the HD 600. I much preferred the HD 600 and enjoyed them for quite some time. It was during this period that I had another opportunity to listen to the HD 800. Well, I think you know where this is going...they were astounding! What happened to that shrill, bright, tinny sounding headphone? Obviously, nothing changed except for my perception.
I see, and agree. Everything is relative. And some aspects of sound you don't notice until you compare it to something else.