It’s notHow badly designed are Naim amps that this is necessary?! If indeed it is...
I've never known a solid state amplifier require a warmup period of more than a few seconds. If the standby current is measured in the hundreds of Watts then you're just wasting electricity by leaving it on all the time. A device consuming 290 Watts 24/7 is going to cost £28-£30 per month at the usual UK electricity prices!