Speakers don't "break-in" but during operation and as the voice coil heats up, this can cause the measured T/S parameters of the driver to change slightly. Nothing really significant to worry about unless the speakers are outdoors in cold weather or your house and therefore drivers in the speaker are very cold. Psychoacoustics and the science around the psychological responses of sound perception and ones own cognitive behavior relating to new purchases is a more realistic view on how perceived sound changes occur in the first hours, weeks, months of the ownership of new speakers in comparison to old.
