It won't be easy to find but with a little effort it could be build as a DIY project.
Here's a page with a lot of technical info but no detailed schematic.
https://www.kenrockwell.com/audio/b-w/800-series-bass-alignment-filter.htm
The smart guys here will be able to calculate the values for the 2 caps and two resistors.
There's a thread covering the subject:
https://www.diyaudio.com/community/threads/bass-alignment-filter-for-b-w-matrix-805.235346/
Hugo
Here's a page with a lot of technical info but no detailed schematic.
https://www.kenrockwell.com/audio/b-w/800-series-bass-alignment-filter.htm
The smart guys here will be able to calculate the values for the 2 caps and two resistors.
There's a thread covering the subject:
https://www.diyaudio.com/community/threads/bass-alignment-filter-for-b-w-matrix-805.235346/
Hugo
I waited years for something to come up on the usual sites and then a fellow diy’er sold their B&Ws and sold off a few hand made (according to B&W circuit diagrams) boards. I bought a couple, one populated which I used on some 805’s and the other which is bare new/unused ready to be built up using whatever quality of parts you want to use. DM me for details.