Should Class A Amps be left on??

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

Thank you @jjss49 
@jasonbourne52 
@ebm 
@wrm57 
@georgehifi 
@gryphongryph
@jaytor 
@audiojan 

Thanks for the detailed replies.
As advised I will turn it on hour before listening and turn it off for the night.
The preamp I will keep it on 24/7, Sumo Athena preamp, also super old roughly 35-40 years old. Preamp does not have standby. But the user manual says "where possible keep it on all the time".

One day the setup sounds great, but the next day sounds little bit harsh. I am using JBL L5 floor standers.

@nebulae 

if your equipment is 30-40 years old it is worth having a good tech check them out look for degraded capacitors etc etc... replacing worn or degraded components can dramatically improve the sound of a good old piece of gear

good luck

Thanks @jjss49 
Sure I will do that.
Thanks.