Leave an amp "on" permanetly

A Dealer Man I know says that, in his experience, tubes take an hour to stabilize and sound good and solid state takes 24 hours.

So a good "green" compromise is to turn your amp on Friday morning and turn it off Sunday night for a good weekend listening session.

The 'green' says turn it off. The ear says leave it on.
The other reasons do not matter.

If it's an SS amp leave it on. It will always sound better that way. I have an Audio Refinement amp that I have left on for 2 straight years and have had no problems.

when I owned McCormack DNA-125 I kept it on 24/7
kept my DNA-225 on starting on Thursday if I knew I'd be listening on the weekend.

These amps do sound good when they are hot and I did notice especially with the 125 that the best sound is reached after the amp os on for about 24 hours. DNA-225 sounded better in general even cold, so it was not such an issue. But it also sounded better when it was on for hours.

Leaving these amps on 24/7 does raise your electrical bill but not by much. Keeping DNA-125 on 24/7 is about equivalent of a 100w light bulb. If this isn't a killer for you, then leave the amp on. You won't hurt it by leaving it on all the time.

I think there are pros & cons both ways. You say you hear a difference in cold vs warmed up. Do you hear a difference between warmed up vs 24/7? I do a little of both, like with my car. I'm not going to turn the engine off at every red light then back on when it turns green but I will turn it off while I'm eating dinner out. If there's not any difference between warmed up and 24/7, I'd turn it off when there are long periods of non-listening.