going to replace the intensity control with
a 5K linear pot as a variable resistor to control the amount of NFB.
I'm thinking of putting the pot in series w/ the 820 ohm NFBR. Has
anyone done a mod of this type and does anyone know a reason I
should not do this mod?

You might find that the
bottom is a little loose after doing the mod(with the pot turned
up).....if it is, you can compensate by lowering the values of some
of the coupling resistors.......especially the ones coupling the PI to
the output tube grids.

I'd go with a 25k pot

you might want two frequency selective feedback controls,
presence and resonance.

 I think I will try going
with a 25K in series w/ the 820 ohm and put the
pot in the external speaker jack.