Don't know about that particular one, but it probably is just a pot (I used to use the input/output volume on my rack f/x to control the volume via the f/x loop). If you have a post-phase inverter f/x loop (on a Marshall, anyway), it will give you a little more "something", but it's not the same as power tube distortion.
There are power-scaling solutions out there that do more, but I'd say get a Hotplate or Weber MASS attenuator (or find a good DIY attenuator schematic, if you're so inclined) if you want to get the power tubes working for you.
Mike