Certainly there is a "player" aspect to the break-in process. One of the hardest times in my professional life was in the early days of the Sterling euphonium development. Sterling would send me a new prototype a few times a year, at which point I would return the one I had and play the new one to evaluate the changes. I was constantly getting used to new playing characteristics, even though I was not changing brands.
However, horns DO break in. My best example was when I was buying my first Sovereign 967. Besson sent me one, and I simply could not make notes speak lower into the 4th-valve register (like the C, C#, D below the bass clef staff). But after a few weeks I was getting the notes to come out reasonably comfortably. At that point you could have convinced me that it was the horn or that it was me. But I didn't like the over playing of that one and asked for a second sample to be sent to me. The experience was identical regarding the low notes! On the new sample I could not make the low notes speak, but after a few weeks they got better. So at the point when the 2nd sample arrived, I was used to the characteristics of the 967 in general (different from my New Standard), and yet the 2nd sample was not "ready" for me.