Resonance control mod:

This mod was first posted by Mark Cameron. I have used it on Fenders and Marshalls with great results. Basically, it is the opposite of a presence control, allowing you to tighten up the bottom. I will try to talk you thru the wiring. 
Looking at the back of the 1meg audio pot, left to right, numbering pins one two and three. Disconnect your feedback wire from the output jack and solder to pin three.
Solder a new wire from the output to pins one and two.
Solder a .0047 cap from pin three to pins one and two.
That's it! I use the master volume control on Silver-face amps, which is a 1meg audio taper.
Good luck! 
James