We should just start off with the small issue of there is no such thing as burn in, my KGSSHV still sounds exactly the same as the first time I powered it on two years ago. This is true of all the amps I've built (and there are quite a few of them) plus what on earth should change at 250 hours? The capacitors finally working as they should after 45 million charging cycles? This is just ludicrous... I also have a hard time believing any manufacturer would ship out any product that hasn't been run for at least 100 hours and gone through many thermal cycles.
As for stock tubes, same thing applies to the BHSE and the SRM-007t so how can that be a factor?