Should Class A Amps be left on??

Sean, the 5 is in a basement on concrete, underpad & carpet and marble slab with nothing surrounding it for 4'. Airflow, cool air is as good as it gets. The listening room is 70F or cooler.
I am confident that the convection factor is as good as I can make it.

Leave them on...most decent Class A amps are designed with this in mind...if you have reservations....and want to turn them off...give them 10-15 minutes of "warm UP" when turning back on...and dont stack other componets on top!
If you have the old MF A1...you can make some chicken fajitas while listening to Mahler..awesome!

continuing this conversation after a time gap of roughly 19 years....

I recently acquired a Sumo nine class A power amp and Sumo Athena preamp. The preamp has a on/off switch that works but the powr amp on/off switch is broken. I currntly have them plugged into two independent surge protectors. I turn them on an hour or two before listening. 

These Sumo amps are 40 years old. I have no idea of their history.

I want to leave them on all the time, but of course they do not have standby mode.
I am afraid to leave them on while unattended.
But also worried about turning them on once every day, the sudden inrush current could damage the powr supply in the amps.

Any advise what to do in this sitatuion? Are there any reaaonably priced slow start/safe start for home audio? I only find lots of different options for surge protectors and power conditioners. Do these surge protectors and power conditioners protect against sudden spike in current while turning the switch on?
Thanks.

A class A amp has a pretty high steady state current load on the power supply, so the caps have a fair amount of current flowing in and out all the time when the unit is powered up. I think this puts a lot more stress on the caps than a couple of cycles during power on.

Most well designed amps will have some kind of soft start circuit which will reduce the start-up current during power up. This is to protect the transformer more than the caps. When a transformer is first energized, it can draw a LOT of current until the magnetic field in the transformer is established. I don't know whether your amp has a soft-start, but assuming it does, this will also limit the initial current flow into the caps. 

So, from a cap lifetime perspective, I think you are way better off turning the amp off when you aren't using it. 

A surge protector is not the same thing as a soft-start. It is designed to protect against over-voltage on the AC line, and will not help during start-up. I have not seen a soft-start circuit built into a separate enclosure, but such devices may exist. 

I don’t think there is any need to have a class A amp on permanently, after 15-30 min it should be 95% there sound vise, and after an hour I would not think it would get any better.