I'm not sure what you mean by "no gain stage." If the amp had no gain stages, it wouldn't amplify the input, and you'd get no output. I'm thinking that you might mean no master volume control but, as I said, I'm not sure.
In fact, the old tube 1200S has two gain stages. The volume control is between them. The amp is typically very clean, and most players find that they take pedals extremely well. However, that seems not to be your experience.
One thing you might try is a clean boost pedal. With that, you can increase the input level to overdrive the first gain stage of the amp, then use the amp's volume control as your master volume. That would give you only one stage to overdrive, which might not be ideal, but it's worth a try. A more invasive method would be to have a master volume installed later in the circuit, but a lot of people don't want to modify their classic amps, which I fully understand.