I have done a very basic setup with bi-amp and active crossover on my 4365s. In my ears the sound-improvement is big in my system compared to stock crossover and single-amp. The sound is subjectively more clear and dynamic, and much more bass the way I like it.
It is very fun to test different settings, and hear and measure what happens. This is to me the best way to learn.

Equipment:
JBL 4365 with temporarily bypassed internal crossover.
DBX Driverack 260, internal output gain jumper lowered to its lowest setting.
Oppo BDP-105, xlr output 4.2 vrms at full volume. I use this as preamp with the digital volume control in the ESS Sabre dac.
DBX RTA-M microphone.
Focusrite Scarlett 2i2 sound-interface.
REW analysis program, with DBX RTA-M generic calibration file
Hypex nCore nc400 6-channels (DIY). 3 channels with about 12Db lower input sensitivity/gain by removing resistor r141. 26 Db, respectively about 13 Db, gain. I use the high-gain channels to drive the LF, and the low-gain to drive the HF.

I know there are a LOT to work on and improve, but my initial settings sounds surprisingly good!

Measurements done only in one position, the listening position. I plan to sum more positions later on.

2-way at the moment. Hence, the UHF is not in use. I keep it simple for now.

Work on delay in the crossover-region is my next planned step, and off course more work on the crossover region overall. I plan to do it with close microphone, and as far away from the walls as possible. The speakers are huge, so I can not do it outdoors.

48Db slope in the crossover sounds slightly better than 24Db at the moment. I think it is best to keep the crossover narrow until it is finetuned/more correct.

The system is dead silent at idle, even when putting the ear to the horn. I am quite impressed by this. No noise at all.

Comments and tips are welcome, I am a beginner at this. But try to keep it positive, I do this for the fun of it!

Settings, measurements and voltage drive of the original network will follow.

Other owners of 4365, please post your settings if you go this route! I know goophy did active crossover as well.