Just wanted to post a follow up to this.
I remembered that the tone stack on this circuit is basically a Big Muff style tone circuit using a high pass/low pass setup. I recalled seeing some info on modifying Big Muff tone stacks for different eq curves.
I found some good info here;
Coda Effects - Big Muff tone stack and here
Metro DC Music. The pages are primarily about dealing with the midrange of the stack, listing values used in different Big Muff circuits that have mid scooped or mid emphasized sounds.
I used the values for a "flat" mid sound. I checked them with the Tone Stack Calculator and they show a pretty flat line and the response from the tone control is much more conventional.
R15 - 39k
R16 - 39k
C11 - 10nF
C12 - 10nF
I'm not suggesting that these values are right for everyone, but this pedal is MUCH more usable for me now. I socketed the four components to be changed and tried 2 different value sets.
I just wanted to encourage anyone struggling with the tone of this circuit to look into these mods. It has definitely helped mine. The best thing about this circuit might be that it uses the BMP tone stack.
Later...