Yamaha CS80

> Has the iG00156 a Butterwoth response or else? Are there specific 
> nonlinearities on the spectrum?

Two separate blocks with one pole-pair each.
A block with just one pole-pair can be anything, from real-pole to bessel to 
butterwoth to chebychef, just depending on the resonance (Q-factor) setting.
The description by naming the Q factor is more common, and in case of 2-pole 
blocks it's _enough_ to completely describe the filter character (alongside 
with cutoff frequency, of course).
Looking at the whole thnig as one four-pole BPF, you can certainly realise a 
bessel, butterworth, or cheby BPF (which means keeping both resonance 
settings the same, and varying the distance between the cutoff frequencies 
and the - linked- resonance of both partial filters). But these are just a 
few selected cases - obviously you can create a lot of unsymmetrical BPF 
responses which won't relate to any of the classic approximations, by 
choosing different Qs for both blocks.

JH.