In my experience, many electronic components show changes when first powered up in a circuit. Most often it’s subtle to barely noticeable. Sometimes it’s clear. It takes minutes to a few hours to resolve. Maybe it’s caps forming. Maybe it’s because this is the first time these devices have been run at operating temps/voltage/current flow. I don’t know and I have not seen a pattern (not enough repeat projects).
I’ve heard it most often in kits, because, I presume, that I’m the manufacturer, and most of the devices have never been in a circuit before and there are lots of new devices interacting for the first time. I’ve heard it in repairs, too. I’d be surprised if it occurred in fully assembled, retail devices. These should have been tested for hours or days before they received final approval at the factory. Customer returns are a really bad thing.
As far as taking hundreds of hours to burn in, I’m skeptical. Human hearing memory for details over time is very questionable. It’s more likely that our brains adjusted to the sound or something environmental, psychological, or physiological has changed that subtly altered our perception rather that actual, noticeable changes over such a long time.
I do know that caps change over time. I’ve done recaps on devices with 40 to 50 year old caps and heard massive improvements in SQ. But that’s a long time horizon. I’ve also done recaps and heard almost no change in SQ. Mostly this can be attributed to how close to spec they are after 50 years, but could also be a magical 400,000 hour burn in period.
