Valve guitar amp head with headphones only
You're right about the danger of running the tube amps without a load. 

During "Silent Practicing" you can disconnect the load. Don't worry, your amp has a built-in dummy load. And when you plug a speaker cable in the output jack the amp output switches to the actual load (speaker) from dummy load.

But never leave the speaker cable having one end plugged in the output jack and the other end unconnected to cabinet.