If you're talking about one of those things you plug in the external amp jack to progressively short out the preamp, looking at the schematic it would probably work, but I'm not sure it would give you what you're looking for (if that's massive overdrive).
Schematic: Invalid Link Removed
Old-school amps like the B-25 weren't designed with a lot of gain in the preamp section.
A post phase inverter master volume would be a better choice as it allows you to force the full output of the preamp into the phase inverter tube. I added one of these to my former Fender Twin Reverb and it worked well.