Can you leave class D amps on and not ever turn them off?

FWIW, the BEl Canto ref1000m amps draw a lot of current when powered up.    If I power both up at exactly the same time, my circuit breaker will break.   Once on though there is never an issue.

So a major power draw event like that is likely best kept to a minimum and is an indicator to me that its best to leave them on if possible.

My Class D Lyngdorf stays on all the time and sounds much better this way. Big difference. I always leave it on unless I am out of town for a week. At least with my amp there is a night and day difference. One hour did not do it. I have had the same experience with other SS amps. 

Regarding sound quality when leaving on versus turning off,   I suspect leaving it on at it should be only helps but have not been dissapointed either to-date after firing it up cold.

Can you leave class D amps on and not ever turn them off?

Yes.

I don’t trust SMP power supplies, they are finicky to component (caps etc) aging

Most Class D’s use smp (switch mode) power supplies, they do go up in smoke a bit more frequently than linear ones, and can be a fire hazzard if not at home.

Cheers George