Burn in is a reality. My 7 days old amplifier is only just beginning to open up. Before that it sounded grainy, closed in, ill defined. I have experienced burn in with every new component, and also with each tweak ( I replace capacitors and such).
I’m seriously worried for people who don’t hear it 🤷♂️
playing music is generally enough but I like to speed things up a bit by playing those 10 hours long punk noise tracks on YouTube. Pink noise is good for burning in electronics.

