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? 

Stand by is fine, but if it's over 20 years old you'll want to have it recapped.