Just for reference Magnepan use to sell a passive crossover like this for the MGIIB's I had, for biamping. Was a single series connected coupling cap for high pass with the mid/tweeter panel and just a volume control for the bass panel to adjust levels. Worked well with my setup at the time.
When the original Magneplanars (model 1-U; three 16" X 6' hinged panels per side, making them 4' wide by 6' tall) were introduced in 1973 , Audio Research was distributing and marketing them, recommending them for use with their amps. The speakers had a built-in passive first-order cross-over centered at 1500hz, one panel for 1500 upward, the other two for below. ARC offered a separate little box for bi-amping the 1-U's, the PC-1. It had a four pots, one for each range for both channels, 1000hz being the x-over frequency. Using it meant the addition of extra connections and cables, so people started putting x-over components on the input jacks of their power amps, the values being determined by the amp's input impedance and the x-over frequency desired. Still the best way to bi-amp, providing there is a way to adjust levels.