The Serge VCFS is a legendary Serge filter that offers unique dynamic control of amount of filtering, from barely perceptible to highly resonant. 
Here are some sound samples of the Random*Source version of the filter, a licensed and authorized adaption of the VCFS for Eurorack. It is an improved design that uses 2 high-end THAT2180 VCAs and features an added Q knob, which allows to increase the resonance for an even wider sonic range.
Check out also https://www.youtube.com/watch?v=HmYNxwNDJ2o
Download the 24-bit files to avoid compression.

Serge Variable Slope VCFS for Euro

Random*Source