I have one of these, it's a really unusual and useful bit of kit. The HPF does the usual thing of progressively filtering out unwanted low frequencies, very useful but can sometimes leave the bottom end feeling a little lacking. By adding in a little resonance you can not only tighten up the bottom end up but also retain some real low end punch. The mid speaks for itself, really handy to have, particularly if you have a bass with 2-band eq (or passive). The LPF does the reverse of the HPF, and in combination can emulate a cab sim. By itself it's useful for filtering out unwanted clacky high end or tuning in a vintage vibe. But with the added resonance control, it effectively works like an Alembic filter preamp (or ACG/East, Lusithand etc). By adding a bump at the cutoff frequency you can tune in a vast array of sounds from any pickup. A clever little box of tricks.