This issue has been discussed many time, but, it never hurts to revisit it.
I had an experience a few years ago. In my system, I have Martin Logan Monolith III speakers. I also use a Krell KBX electronic crossover that is configured with wave shaping circuitry for the speakers. The KBX is solid state and doesn't have an on/off switch. So it is typically always on, unless I unplug it. Well, I noticed a burning smell. barely noticeable at first, but it got to the point where I had to find out where this was coming from.
It turns out that this KBX unit is definitely class A, totally discrete circuity and extremely well designed (circuit wise). But, that sucker gets warm, and being left on, definitely didn't help.
I traced the problem, reversed engineered the schematic and repaired the unit. But, I told myself that I would never leave electronics on all the time in the future.
Not only do I not want my home burned to the ground when I get home from work, but It just didn't make sense to me to leave this equipment on when not in use.
So, I unplug the KBX after uses, turn off everything else.
I noticed that after about 30-45 minutes of warm up, my system sounds great.
I have solid state equipment and tubed equipment and no way on this planet will I leave the Audio Research REF 250 amps on all the time. With about 2000 hours typical use on those very expensive tubes, that is a non starter.
The Audio Research Pre-amp, Phone stage and DAC also.
The Mark Levinson 23.5 amp that I use to power the woofers on my speakers gets warm also. I recapped the unit a little while ago, when I decided to get it serviced. But, being an Electronics Engineer, I know a little about heat and how it affects electronics.
Other than my refrigerator, I really see no reason at all to leave equipment on when I'm not using it.
But, to each their own. I learned my lesson with the crossover. I guess I could install a power switch, but it is just as easy to get up and unplug the sucker.
Now that I'm thinking about it, my Transparent Audio Power conditioner is always on also. Hmmmmmmmm. need to think about unplugging that unit also.
It is starting to get a little old having to turn everything on and off. Plug the KBX in first. Turn the pre-amp on, turn the phono stage on, turn the turntable power supply on, then go and turn each amp on. Lots of walking around and bending. I know. It sounds silly and lazy complaining. But, I guess it would really suck coming home to a house fire.
Just thinking.
enjoy