esp since you're not just turning on your amp are you? If you're listening to music, then you need a source, an amp, some cables and loudspeakers or phones as a minimum.
How do you know it's the amp which is making the alleged change to the sound as time goes by? Why not the rubber surrounds of the speaker cones warming and softening with use ( more likely and quite measurable with sensitive enough kit I'd guess)? And for vinyl lovers that cart is NEVER at 100% in a cool room on the first album, but once it gets to 20c + (warmed by your amp below) then it's a whole new sound, but amps????? What's happening, and if a dealer here knows I'm wrong please use your expertise to explain exactly what is happening inside the amp? Graham must know surely. Baited breath and all that.
PS. I'm not including valve amps in this. Valve amps do need some time to settle down. Maybe up to 20 mins from turn on each time but that's another discussion.