Hi Matthias,

are you sure the manual page's crossfeed parameter for level 1 is correct?
1: 650 Hz, -13 dB (just a touch)

I program my DSP and cannot match your setting.
I can, however, match level 2-5.

For instance, for the high pass filter, below the red line is your FR and the blue line is mine.

For bs2b algorithm, the high pass filter's gain is level / 6 - 3. now put level = 13, the gain is -0.83

seems your gain is close to -0.75.  Maybe you mean
1: 650 Hz, -13.5 dB (just a touch)
?