One reason is usability:why we cant get freq down to zero with this filter
BiFilter uses logarithmic frequency controls. Only a musically usefull range (in my eyes) should be possible.
The second reason is math:
Some filtertypes uses buffers for calculation. On a cutoff like 0Hz these buffers needed infinite size.
The third reason is continency:
All filters / effects should have the same sonic range. This makes them easier to compare.
If you think that 40Hz is not low enough, you could try other products.
Regards,
Markus
