With the Lamm ML 2 amps, you can definitely hear a difference as you play music for the first 45 minutes-1 hour. A gradual opening up in almost all aspects. If it is something else, then it must be the tube line stage or the tube phono stage, or perhaps the cartridge; they are part of a system that noticeably improves in sonics after the first hour of playing. I think it gets better by smaller degrees after that, but it is noticeable, and repeatable. I’ve been using these amps for about 11 years. And many previous amps I’ve owned, all tube, were the same in that they changed sound character for the better after warm up, though perhaps not as dramatically as the Lamm.
The Lamm line stage I had- the L2- had a solid state audio circuit and tube power supply, and Vlad recommended keeping it always powered on; I did that (though I never really liked running tube gear when I wasn’t home). I would pull power during electrical storms, and it seemed to take days for that thing to come back on song after it had been unplugged for a while.
I gather that solid state gear benefits from warm up too, but have less ownership experience in high end solid state gear. Switched to tube amps and preamps around 1974 and have soldiered on with various good tube products since then, none of them poorly made or designed- mostly early mid era ARC stuff.