Seems like the best "upgrade" for tube amps....

Yes, it is true. Electronics do need some time to stabilize. This is especially true for tube gear which uses filament to heat up the electronics so after some time, they sound better.

Happy Listening.

Agreed. Tubes take some time to reach their best sound. I always find that music sounds better after the amps have been on at least 90 minutes or so.

Not day and night, but just more involving and enveloping.

You should turn your rig on and listen to it IMMEDIATELY before it gets too warmed up, and then turn it off after a few minutes and go away. It doesn't necessarily sound better, but this forces you to get out more.

Service people that worked on tube equipment (TVs) back in the day have suggested letting tubes warm up before putting signal through them. Supposedly this helps protect and extend tube life. I wait at least 20 minutes after turning tube amps on before playing music. Independent of that, things do sound better to me about an hour into a listening session.

Isn’t it true that speaker cones also are designed to operate best at a certain thermal equilibrium point? Most often I let my whole system warm up prior to a listening session. Does sound better after an hour.