In part because the temperature difference between on and off is so great, the sound does change as the components warm up. (Obviously, designers work to optimize the sound after warm up, not during.) Keep them on, and you won't have to wait for the amps to warm up for an hour and stabilize every time you listen.
Should Class A Amps be left on??
Ok- Audiogon members need some advice here. Should Solid State Class A Amps be powered all the time? Have heard two school of thoughts on this. Apparently from what I have heard this is a moot question for Class A/B amps due to the usual low biasing of A/B Amps. From what I have been to learn so far Class A Amps should be left continously on.