Low pass filter cutoff frequency cannot be edited from script. The high pass filter member with the same name is editable, and the only difference between these two is the custom editor slider on a low pass filter. Perhaps I am missing something, but it looks like a bug.
Also, I made sure that this parameter is not controlled by curve. Any help or insight into this matter would be appreciated.
Ok, so I kinda solved my problem by implementing a custom low pass filter. Here are the sources:
http://stackoverflow.com/questions/12990423/audio-clicks-pops-from-filter-code - there is an implementation in the answers - this one sounds very much like a default unity LP filter
http://baumdevblog.blogspot.com/2010/11/butterworth-lowpass-filter-coefficients.html - this one is not resonant, and is perhaps more effective for common LP filter uses, but has a maximum cutoff frequency at around 8.5 kHz (could be a problem with my implementation, but not likely).
If anyone had stumbled upon a more effective LP filter code, please share. 
I have the same issue, I really don’t want to use a workaround for an issue like this. Any help?