Non-solid-state electron systems should be expected to have their own idiosyncracies, regardless of whether superstition is building up faster than it is being dissipated.
The regular ordinary procedure for cold-starting high-power vacuum tube systems is to switch on the unit while the tubes are in standby.
This will allow the cathodes to heat to their characteristic glow, fully developing the ability to deliver the current you will soon be demanding.
But no high voltage is applied to the anodes (there will be no sound from audio systems) until removed from standby.
A minute or two later is good.
If the cathodes are too cold when applying the high anode voltage, especially at high current demands, it can strip the cathodes of their ability to provide the full performance when you need it most in the future.
Small tubes, most preamp types, run on lower voltages and currents, don't suffer noticeably, and were cheap to begin with.
It's the bigger more expensive tubes that standby is for.
Shut down by switching to standby while the main unit is still powered on.
After no more than a minute or two, fully power off the unit unless you would otherwise have to cold-start unreasonaby soon afterward. This is about 10 to 20 minutes for popular audio units, or much more if there is a true need for instant-on over a longer term.
24 hour standby is not very good for the tubes either, best fully powered off to save hours, or idling at a reasonable fraction of their ratings.
On standby tubes are still more vibration sensitive than fully cold, so it's seldom a good idea to leave them on standby while there is a source of strong vibration from other sources.
Never leave a tube amp in direct sunlight unless fully powered off. Standby will not save you.
If it is allowed to heat up above ambient for any reason, let it cool back down to ambient before powering up.
Gear that can run continuously at ambient 100degF can die at 70degF in the sun.
Without the sun, different gear (including differing power bias settings on the same gear) will have different maximum ambient temperatures beyond which only short-term operation is advisable, even for units which are otherwise capable of continuous operation under slightly milder conditions.
For musicians it's the class of thick-cone reinforced speakers that can take 20 or 30 minutes (near their power rating) to break in every time, that can throw you off.
Speakers can not be "warmed up" silently, you have to make some noise.
After this cranked-up break in, reach into an open-back cab and you feel how hot the magnets are, they were made for this but not in the sun either, especially closed-back cabinets where you can't touch the magnets but you know they're even hotter.
But often it's not the warmth of the magnets (or the tubes) that makes a (beneficial) difference in the tone, it's the preflexing of the cones which makes them more articulate afterward.
Most amps themselves warm up way faster than that, and can do it silently.