Well, not exactly, Im not a schematics expert, but it would be like a second master volume. Yes you can turn your master volume to full tilt and use something in the fx loop to keep the volume down..but tubes drive into saturation by the volume going into them, so if you attenuate the signal at the fx loop, you get less ummpphh (its a technical term ;) ) going into the power valves..so they wont get that sweet power overdrive.