In the General 2nd Order block, you can select a Bessel LPF. Is this digital Bessel filter generated using bilinear transform or some other transform? As I understand, the bilinear transform does not produce a true linear-phase filter:
Note that the digital Bessel filter is designed using BLT. That introduces a phase response error due to the frequency warping. In the other words, the linear phase will be considerably distorted if Fc is higher then Fs/4 approximately. You should use impulse invariant approach or other phase preserving technique to design Bessel filter for high frequencies.
The only advantage of a Bessel design over a Butterworth or Chebyshev is that the phase response is nearly linear throughout the passband, i.e. the group delay is almost constant throughout the passband. The ``warping'' inherent in the bilinear transform method upsets this linearity.