If I wanted to get rid of the tone stack I would remove the Bass and Treble pot and also C10, C11, R11, R15. I would leave C7 and R13 and put a bridge across R15, I would adjust R13 value so when the volume pot is on full you have the maximum output you want. R13 and the volume pot forms a voltage divider and lowers the signal reaching the pot.
C7 isolates the output signal from the DC potential but lets the signal through, if you want the maximum signal available then just place a link across R13 and R15.
Cheers
Mick
Ps....that is just my uneducated assumption of how it would work best.