I've encountered this problem frequently, and here's a sure-fire solution:
First, go to a hardware store, purchase a 10-32" hex or round die and holder and its matching 10-32" tap and holder. Apply a good light oil to the areas front and rear of the small stripped hex nut(s) all the way out to the tip of the lever rod. While securing the tone control handle (or round knob) in one hand, manually and slowly turn the die clockwise and thread it slowly on to the tip end of the lever (the tip is usually slightly flattened on one side but just ignore that). Turn the die forward in quarter or half turns, unscrewing it a little and reversing direction after each forward turn, then repeating this process. Once the die stops against the frozen hex nut, back off the die and try unscrewing the nut again - sometimes the oil alone will free it enough to allow it to move forward and connect with the newly rethreaded section of the rod.
If still frozen in place, completely unscrew and remove the die. Then get the proper size hex nut driver (hand tool or electric screwdriver mounted) and tighten the hex nut down towards the shell compressing the spring behind it. Just a few turns is all you need. Then reapply the die and repeat the process above, correcting the threading all the way up to stop against the hexnut. Then slowly unscrew the die and remove it from the rod.
At this stage your wingnut should be easily unscrewed and removed using your hex nut drivers or fingers.
After full disassembly it's a good idea to completely correct the threading of the hexnuts and threaded rod using your 10-32" tap and die. Correct the entire length of threading on the long lever rod all the way from tip to end of the threading.
After re-working the parts they should apply and tighten into position quickly and easily.
This entire process of thread correction can easily be done in no more than ten minutes! With minimal cost in tools you save yourself countless hours of struggle and frustration. The tools you can keep for all future projects when needed. Best of luck!
It's easy!