If you look at the tubes with power on and stand by off, with the amp set to 50W, the active tube should have a blue glow (if not prominent, look into the slots on the plates (unless you have KT77, there should be slots) and at the center, there should be a blue aura around the cathode. The other tube would not have the blue glow. I looked at the manual (stage I) and it does not indicate which power tubes are active at 50W. Also uncertain which rectifier shuts down.
You should not run the amp without the tubes in the sockets. You can also opt to just swap the inner pair with the outer pair after a few weeks of use. Also the full complement will be there if you accidently move the toggle switch to 100W.