Should Class A Amps be left on??

Pass Labs XA60.5 that I put in standby when not playing music. Yes, they did increase power consumption slightly, but with solar panels, I don’t have an electric bill so no problem. :-)

If your Class A amp doesn’t have a standby mode, turn it off over night. If it doesn’t sound good enough 30-60 minutes after powering on, or into Class A, get another amp.

Turn on 1 hour before listing only.

I could be wrong about this but I thought that class A amps were least efficient when not driven.

This is correct, a pure class-A or very heavy biased A/B still draws the same current from the mains driven or when at idle.
Both amps at idle gives off more heat from it’s heatsinks than they do when driven, but when driven some of the energy that causes the heatsinks to get hot is then diverted to the speakers driver coils instead.
All the pure class-A and heavily high biased class-A amps that I have built, all sounded their best after around 1/2hr of being turned on.
Cheers George

op

answer to your query is no

as others have stated, turn on when listening or a few minutes before

amp will come up to temp pretty fast as they run hot, music should sound very good before long

nice to save some energy too