The Double NFP is a filter-based preamp alike the ones seen on basses like Alembic and Wal.
It features one filter per pickup, offering more tone-shaping options, that are actively mixed through the pan/blend potentiometer and into a Master volume potentiometer.
Being a low pass filter, it lets all bass frequencies pass through up to 4Khz where it starts cutting. The cutoff point has a resonant peak that can be boosted by pulling up the potentiometer.
Each Filter potentiometer also allows you to sweep the cutoff point(resonant peak) within a range of 150Hz to 4 khz.