List of most high end analog active crossovers ever made.

Apogee utilized a "Dedicated Active Crossover" dubbed the DAX for its Diva speaker (large, planar magnetic speaker, sort of like Magneplanars).  I don't know about the specific contribution of the DAX and biamping, but, the system sounded quite good.

I used a Marchand XM44 when I had Castle Winchester speakers and bi-amped them. Perfectly satisfactory, at least until I discovered ESLs!

LUXMAN A-2003 Electronic Tube 3-Way Crossover!

Mike

The B4 is unbelievably versatile: high-pass and low-pass 1st/2nd/3rd/4th-order filters in 25Hz increments from 25Hz to 6375Hz. All discrete (Toshiba Jfets), no opamps or IC’s.

I have three speakers and would leave the tweeter passively crossed. But on a scale of 1 to 10, how hard is the B4 to use for beginners? Would the learning curve and/or functionality of this one be a easier? 

https://sublimeacoustic.com/products/k235-stereo-3-way-active-crossover