This is interesting topic. I do not really believe in speaker break-in, but I've had two cases where the speaker has changed significantly after some time.
First case was a car subwoofer years ago. Initially it did not get very loud, but when played for first 30 minutes I had to stop the car several times to reduce gain from the amplifier as it was getting too loud.
Second time was my Wharfedale diamond 12.1's. I absolutely loved the sound they had when first played. I was really surprised that I could place them really close to the wall with zero boominess. Then one day, maybe after two months of having them I started listening and there was suddenly way too much bass. I thought the kids must have turned the subwoofer volume, but when I checked the sub wasnt even on.
I then remembered that last time I listened those speakers I ended the session by testing if the preamp speaker type options had HPF, by playing 40hz sine wave and seeing if there was any change (no effect). That might have been the first time the woofers got any real excursion and maybe that was what made the difference, but nonetheless there was definitely change in the speakers. I had to move the speakers away from the wall and use dsp to cut the lows after that.
Just wanted to share the experience..