"Burn in" believers imply that something will change for the better after a period of time.
The only way this can happen is if the characteristics of the components change.
All good equipment is measured, tested, calibrated and designed to perform it's best before it leaves the factory.
If any of the above characteristics change after this with "burn in". Then this equipment will not test the same, be in calibration, or measure the way the designer wanted it to remain when it left the factory. And could only sound worse after "burn in".
Ok lets say things do change after "burn in", how does any designer compensate for this, in his calibrations, measurements, tests and design????
The only thing in audio I know of that can change "sound" for better or worse, are mechanical things like speaker suspension systems
Cheers George