A small augmantation - I designed the circuit to have a -4dB loss in the midband at all settings. The bass and treble boosts are relative to the nominal -4dB output and are independent of each other.
If you wish to remove the Fix entirely to compare, then you should make the 4dB level adjustment as Doc B described. However, with the Fix in place, you can set both bass and treble to zero dB boost, and make a midband-valid comparison to other settings.
At the flat setting (no bass or treble boost) there are no capacitors in the circuit - only resistors.