That cathode magnatone feedback rig i think used to be pretty common in old record players and radios, it's main aim is to use as few parts as possible to have at least some feedback for noise reduction. Feedback in tube amps is mostly about noise reduction, where in solid state stuff the whack transfer characteristics require feedback for more reasons. I could possibly find in my textbooks the math to apply, but it would take way less time to just temporary solder a wire jumper or two and see what happens. The best tone stack solution I believe for this simple amp would be any one-knob tone put just before the power tube grid. Maybe even just a high cut, just like a guitar tone control. The way to control the amp then is turn it up till the bass falls apart, turn the guitar up some then use the amp tone to shave off the harsh highs when distorting hard, then turn the guitar down some for good clean or crunch, and have some volume knob left for solos or something. The old way of things!