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

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).