Get the best crossover you can get and don't look back. The mid-bass struggling to reproduce deep bass and midrange at the same time will thank you. The amps will also have more overhead and reserve not having to do low bass while the 1500-3000w dedicated sub amps will be happy to pound the snot out of the woofer of the sub.
There is no precise crossover setting either, since the suckout of the speaker in your room will depend on how far apart your speakers are and how far from the walls they are as well. There are actually formulas for that, 1130/d x 0.3
where the d is the distance from the center of the woofer to the nearest boundary. There is inter-woofer interaction as well between the two speakers. When calculating the suckout notch between woofer (or subwoofer) centers you would use half the distance (1/2d) between them as d. You would still use the 0.3 multiplier.
If you measure the response of your speaker in the room and the suckout happens to be in the range that a sub like JL can handle, it is big help when you have an active crossover and you can eliminate the speaker from the dip equation. Trust me, the JL subs do bass no worse than any speaker out there, including Sonus Faber....
That is what I did with my SF and 2 JL subs and an active crossover, the results were well worth the effort, providing you understand the setup and what's going on.
Pretty typical in room response of a speaker, look at the bass...
