Better sounding after 1 hour

My ATC integrated is SS and like the other posters, I think it sounds better after about an hour. I read it explained somewhere that this is because the final factory biasing is done after bench output testing, when the amp is hot. Don't know if it's true, but it was one of the more plausible explanations for the effect that I've read.

All electronic component parts operate best (at their desired specs) when they have reached thermal equilibrum. I will let my system play for 30 min or more before begining a session.

Yes. With tubes is it always the case. It depends how long it takes, if the tube amplifier has an automatic bias it is much shorter.

Tubes 1 hour; solid state 2 hours.

I leave solid state parts of my system on 24/7 and I can hear a difference if unplugged for a bit to swap something out and then replugged. It really takes a full day to get everything sounding good again, especially toroid transformer gear. That's not scientific, just my experience. Tube gear sounds good after an hour but great after two or three so I turn it on when I walk in the door and then by the end of dinner, listening is wonderful.