Leave an amp "on" permanetly

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.

my nuforce ref 9 se amps stay on all of the time. no problems

I will say that it is possible for a switch that is turned on and off will eventually give out.