I have no experience about headphone break-in...
I dont negate his existance though ...
Each component need some optimizing working time...
My AKG K340 headphone need warming time to be optimal... Because there is a transformer and a crossover inside... It takes 10 minutes to reach their optimal... Very audible...But warming is not break-in...
But i never experiences break in with headphone and i dont have any opinion about it really because material break-in and psychological adaptation play together and it is very difficult to separate one from the other especially when a new product will be experienced ONCE only before and till AFTER he reach his optimum or till we will adapt ourself to it...
I experienced break in with an interconnect cable... Once... Very audible... Thats all...