Leave it on?

In most cases this is just picking nits. Well-designed gear that is relatively low in idle power draw (preamps, class D / G / H amps) should have NO problem either way. Do what you like. Gear that is high in constant power draw, are therefore heat dissipation - tube power amps, class A and high bias AB solid state power amps - should really be kept OFF when not in use. In my experience, hotter components are also more likely to sound good quickly after warmup - simply because they hit thermal equlibrium quicker. SS is always slower to warm up than tubes.

@grannyring ”What does this have to do with the topic?”

l think you missed the point. My point was not leaving electrical devices on unnecessarily. 
Refrigerators left on yes, the nature of the beast. Every electrical appliance is a risk factor. In London a refrigerator caused a fire in a housing complex, Grenfell Tower. 70 lives lost that night.y

Yes again with global warming, arguably self destruction with the jury more in than out. That was only part of the debate regarding wasted power, it’s not all cleanly generated like wind or hydro-electric.

@mulveling Good points posted on what you believe “should be kept OFF” 👍

@mylogic I understand your point. I suppose life with our technology and conveniences comes with risk.   More risk than life before them some 100-150 years ago? Different risks for sure and obviously less overall.  I just don’t see the added risk of keeping your Class D amplifier on. Class A SS amps don’t all run hot today. Valvet is one example. I would not leave tube gear on 24/7. 

 wait till you learn that we can’t even measure the Earth’s temperature accurately

maybe unlivable and deadly hot in Europe right now is not accurate for you, but counting the number of dead is a precise number

@grannyring ”conveniences come with risks”

I note your points, thank you. No offence intended.