> standard treble control is a simple RC low pass filter with R being a
> pot.
>
> What about changing the roll off rate instead or as a combination with
> the corner?
Like, using a blend of -6 dB/octave highpass and 0 dB/octave allpass filters, with the potentiometer to blend the two? The 'simple RC low pass' is NOT the usual audio 'treble' knob , because the treble control has both boost and attenuate settings.
Look up some more sophisticated audio equipment; their tone controls and graphic equalizers are rather more sophisticated.