Our brains do a lot of correction, making sound into what we expect to hear, given our experiences. Some day we'll see how hard it works via brain scan. That could be a key measurement in ear fatigue vs relaxation.
My analogy is another sense - Vision. I wear a not unsubstantial prescription that includes near sightedness and astigmatism. With a new prescription or a change in lens material or even just a regrind, first tryout is always disorientation. Peripheral vision is warped with straight lines noticeably curved. After a few days everything appears normal again. That's my brain at work "getting used to it". Tests say I'm 20/20 corrected (or so). That's the measurement standard all prescriptions are judged by. No test measures how hard my brain works to "get used to it" or why everything looks more natural, atmospheric and relaxing when I take off my glasses.