Slaphead
500+ Head-Fier
Yeah, that was probably just you adjusting to the treble. I find the same thing happens to me every time I switch back to the T20 from a different IEM. Is that to say that they un-break in once I stop using them? No, simply that I was no longer mentally adjusted to them.
I initially went down that line of thinking. However I also spend a lot of time with the Beyer DT1770 Pro and the Beyer DT1350, and when I switch back to the T20 the initial hardness in the treble is definitely no longer there.
Of course brain burn in is still possible in that my brain may have a T20 mode burned into it if you like, but I find that a bit far fetched.
That said none of the HPs I regularly listen to have what I would class a recessed treble, so I would guess if you spend some time with HPs or IEMs that do have a recessed treble, and get used to them, then switching to the T20 is likely to be a bit of an aural shock.