Charles,
Thanks for your interest in the Percolator. I hope you enjoy building it.
The NFB loop is connected from the output to the cathode of a previous gain stage via a resistor (you’ll see the schematic when the kit arrives). You are correct, a good way to mod the circuit is to manipulate the NFB loop with either a feedback level control, some tone shaping (via an RC network), or a switch to disconnect it for extra gain in the midrange.
I’ve done this sort of thing with several different amps in the past, but it’s been so many years now since I designed the Percolator that I can’t remember the specifics of my experiments with it’s NFBL. Feel free to experiment with it yourself!
Have fun and take care.
-Brach