As an old bag who has had a LOT of different stereo equipment over the years, I can say IMO that equipment does in fact sound better after about an hour, give or take fifteen minutes. AND that it still sounds even slightly better yet after being left on a day. give or take six hours.
This has become MORE true as stuff has gotten better, since the 1980s...
i used to leave everything on all the time. My old Forte' 4a fifty watts/ch space heater cost about $10 U.S. a month to keep on all the time.
With an eye to conservation, i often turn off the stuff if i am not gonna listen within eight hours. Since I retired, this is not at all that often.
My old Forte was far more obvious about next day better than my new Bryston 4B-SST2 is.
Since I basically Only listen to music all day.. and all evening with only a few hours of TV if any, the stereo is on all the waking hours.