Do amps sound better when left on for 30 minutes ?

It's actually pretty much an industry standard to leave solid state equipment on all the time. The designers say that the "rush" from turning on and off is more detrimental to the equipment. Tube gear, of course, would burn through tubes too often, so they need the half hour or so of warm up time.

My experience is that they don't warm fully unless they are playing music. I can leave my kW500 on for hours and it still needs playing time before it performs at its best.

I have a tubed amp and preamp and they always sound better after a warmup time of 30-60 minutes.

I have tube equipment and turn them on and put cd on ( low volume ) play for 45-60 minutes..Usually cd is finished and is ready to go.......

I warm up tubes in amps 30-45 min as to not stress the tubes and shorten their life span. It seem after an hour to an hour and a half of playing everything starts to settle in and the system sounds more liquid (homogenous).