No idea about tubes, but years ago I used to own an old Marshall that was the same way & used a thing called an Attenuator. It was just basically a round knob in a metal box that was on top of the amp.
You turned your amp way up past it's normal limits, and used the attenuator to control the volume. No idea what it was actually doing, but it was connected between the amp and the speakers and just allowed you to control the volume that the speakers were putting out regardless of how loud your amp was.