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.
Seems like the best "upgrade" for tube amps....
...is to leave the amps on for a few hours before playing. I've noticed recently that when I turn on my Joule amps and let them run for hours, it makes a huge difference in the dynamics and high end sweetness. It gets hot in our house, but it's worth it. Is there a technical reason for this?