Should I leave an Old Krell FPB 400 cx on all the time?

It's a class A amp and uses a lot of electricity I would keep it in standby when not listening and flip it on 30 mins to an hour before listening.

I agree.  It's not a refrigerator.  Doesn't need to be left on 24/7.  I typically turn my system on about an hour before listening.  Also, for class A amps, that seriously shortens circuit and part lifetimes if left on with that current and heat.

For tube equipment, unless there is a standby circuit, it makes absolutely no sense to leave tubed equipment on. As long as they are on the tube life slowly goes away.  For expensive power tubes, that is a no brainer.

The Krell FPB 400 has a slow start circuit that protect circuitry from power on surges.  So, turning it on/off isn't a problem.  Also as mentioned previously, if it has a standby circuit, that solves the problem.

enjoy

It does have a standby mode but the amp stays a little warm. On the back of the amp is a main switch to turn it completely off. So considering the age of the amp would you goners turn the amp COMPLETELY off or keep it in STANDBY MODE?