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.

Use standby mode unless you don't expect to be listening regularly or are leaving for vacation or a long business trip.

Got 1 to 2 years left before recapping thats why I was wondering to milk a few extra years if turning it off completely will extend life. 

I keep my old Belles 400A on all the time. It has no stand-by. Luckily, for start-up and shut-down for the rest of the system, I have an ifi Mini Tube2 between the pre and amp that disconnects/connects the connection when I turn it off/on, but the amp stays powered 24/7.

While not in use, it cools off and stays cool. I’ve watched my electrical use and no change after many months of doing this. When I start-up the system, the Belles is also ready to go.

As this amp is over 30+ years old, I’ve been told there is less strain on any old parts to keep it on vs turning it off and on daily. So I do. 

Standby is the way to go,  turning the amp totally on and off can stress the caps more than just leaving it on or partially on (standby).