Sallen Key variable filters -> LM13700

[Dimitree]

hello everyone
this is the "External Signal Processor" module of the MS20 synthesizer by Korg.
I'm trying to build it, but avoiding the use of the 4-gang 100K rev-log potentiometers, instead using LM13700s.

http://www.cykong.com/Synths/Korg%20MS-20/Images/MS-20-ESP-Schematics.gif

as you can see there are 4 sallen key filters 2pole filters, a HPF, then 2 LPF, and another LPF at the end.
My first obstacle was calculating the freq. range of the filters.
Also, I don't understand why the last LPF is not a HPF in series with the first HPF.