I recently bought a state variable low-pass filter with variable Q from Helmuth Lemme. Details here - https://www.gitarrenelektronik.de/produkte/aktive-elektronik/state-variable-filter-detail.
High quality electronics and reasonably priced. Intended for onboard installation but I configured mine as an external pedal to have the flexibility to play around with different basses. Works great. Freq range is from 300Hz to 5kHz. Q factor is continuously variable (weak to strong) with a pot or you can use a 3-position mini-switch.
There's also an Alembic style filter called JTEX Distiller made by a chap in Canada available on ebay: