Should Class A Amps be left on??

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.