I´ve got a post phase inverter vol pot installed on my hiwatt dr103, so I actually could hear what´s going on in there without the output tubes saturating at all.
The original master volume goes right after the tone stack, then to a buffer and then hits the PI.
Even being a pretty "clean"amp, you get some crunch from dimming both preamp channels without the PI afecting that saturation. The moment you open the master volume tap enough, you can clearly hear how the PI saturates, (a lot), and how that rounds the tone preparing it for the final blast.
Your idea of swapping parts on the jcm layout to match a sort of a single preamp channel model t, (for example...
), got me thinking that could be a good try.
Anyway, maybe just tunning a simple gain stage, as Travis said, would do something similar.