Burn in is real and noticeable in electromechanical devices, and pure mechanical devices, in which they need a physical adjustment of the parts in movement, or the loss of stiffness as some may call it as well, tha means in drivers, speakers, motors, cars, shoes, clothes, etc...and even though is not that big as many people claim them to be...
Cables, electronic components, etc...regardless of what hundreds of people will tell you, and what hundreds of people will make you believe and argue, there is no one single bit of physical evidence of the existance of such improvement with burn in, other than in thier head and to their ears...What is said in any EE course and in all books, is that any electronic componet after reaching the working temperature, and estabililation of the internal flow of current, is ready to be used, and that is reached inside the first hour of use, unless in very few exceptions...
It will be nice to ask someone if a CPU in a computer requires burn in to work faster, or if a phone requires burn in to communicate faster, or if the Tomahawk (the actual missile) is burned in for 400 hours before being sent to the target for more accuracy (BTW those are militar devices, which electronic circuits are far more complex, and should be, as logic indicates, far more reliable than any audio device) or maybe a new electronic panel on a nuclear submarine, or on a plane...If an audi amp requires burn in the before mentioned circuits will do as well, trust me on that, they will not jeopardize the security for 400 hours of use...