Thanks Alex and Dan,
I am very busy now but I'll look into your solutions ASAP.
I am aware of ggee objects for biquad~ coefficient calculations.
Tough I understand the idea to "split the spectra" of a signal with low-pass, band-pass and high-pass filters... what is still unclear to me is : since the outputs of the filters are then mixed together, how to make sure that overlap correctly so that if the 3 (low, band and high) filters have 0db gain ([*~1]) the sum of the 3 together will be similar to the input (flat response), like it does when your mixer's EQ knobs are all centered...
and what does it really mean when you turn it counter-clockwise, for example "MID knob" to -6 dB ?
I suppose it isn't a rejection/notch filter ? So it's just a bandpass filter with amplitude 0.5 / -6 dB and then mixed with the outputs of the lowpass and highpass filters ?