Look at the Caswell preamp. See the 1ma pot on the extreme right? that is the master volume.
Now look at the Champ II schematic. See the 100kl master volume control?
Get where I'm going?
Step 1: Grab yer Champ II and scoop out everything before that .01 cap.
Step 2: Add Caswell.
Step 3: Test. If there is too much drive, use a smaller master. Go to a 500ka or a 250ka or a 100ka. You can try linear pots if the taper works better.
Note that the Champ II has a split load phase inverter instead of the long tailed phase inverter found on the larger Fender and Marshall amps. The split load inverter will clip asymetrically and do it sooner than the long tail, which can be very cool depending on what you want.
"Understeer is hitting the wall with the front of your car. Oversteer is hitting it with the rear." --"Fireball" Roberts