Hi,
I calculated coefficients of first order filters for recursive filtering using [biquad~] using formulas found in http://www.dspguide.com/CH19.PDF, page 323.
I could only confirm what I was fearing from the beginning : it's not possible to approximate the given transfer functions given in the article no more, as the slopes of the individual filters are less steep. So the filter calculated previously (quartic) is the best that we have now.
My curiosity stepped down and I decided to leave this tone stage as is.
Nau
edit (removed) : joined last version. (0.3)