If you're using an active XO to divide the signal between your Theatre-4 and sub, there's no need for a passive XO in your sub.

I re-read your original post again and you mentioned "active-sub" a few times. An "active-sub" usually has a plate amp (active low-pass and amp) built in, so you don't want to use one if you're using an external active XO and amp for the sub.

Again, the second option of just using a plate-amp is very respectable and economical. Perhaps Bill Epstein can say a few words about his Titanic and Theatre-4 combo.

Gar.