Why manufactures don´t burn in their amps and ...

The tangential discussions are a reflection of the impacts and unease the buyers are subdued to when purchasing new gear. IMHO, we're still on track here.

OK, the Mrtennis comment goes into the Audiogon Hall of Fame.

"this hobby is so subjective that when you have 2 audiophiles in a room, you will get three opinions."

That's nonsense. You will get four opinions.

The simplest answer is that it's a business, you want all of that done for you, then you are going to pay for it! You aren't getting it for free. Component makers are not cable experts and are not about to squander revenues to research for the best cable for their component. Whether you like it or not, that's your job!

I would think most manufacturers have some type of a QA process where the product is burned in for x number of hours to weed out infant failures that occur.
Some may even have enviromental chambers that control temp,humidity,and vibration that affect the product.
Power cords buring in may have some type of merit but I don't know what the audible improvement would be.