my 2 cents on sub integration
i use rels (a pair) and their high level inputs, thus the main speakers are run full range, i place the two rels asymmetrically
i follow the instructions carefully on set up and it always works out well, plenty of speed and seamless transitions (meaning you can’t hear it even if you try)
i agree with ralph, keep the xo as low as possible, i would say with fritz’s probably around 50 hz as a starting point, provided the speaker is set a couple feet away from room boundaries