How to..

[collinsamps]

I would probably attack it from the standpoint that allowing certain NFB frequencies to pass while attenuating others is identical to boosting or attenuating freqs in the circuit 180 out from the NFB settings. Meaning if NFB doesn't affect it adversely then it's not attenuated. You could get to the same place adjusting NFB across the board with a standard potentiometer and adding a tone stack pre master and post last stage amplification to pass what you want to pass. Admittedly more knobs but similar results.