The specs on the 805 D4 state a frequency response that might be -3 dB at 42, possibly dropping further on the way to 34. What you get in-room will probably vary a bit, depending upon room factors such as size or dampening with rugs and furniture. I’ve used a REL T/5x single in two different rooms, and have been satisfied by placing the sub about 18 inches from a corner, and adjusting from there.
I’d start with the crossover set at 40, and then add 6-10 clicks on the volume knob. Then listen to something you know quite well. Play this song, and some others at a variety of volumes. Is it too much or too little for this song? People report best results overall when it’s nearly tough to tell whether the sub is in play. It’s only a starting point, because rooms and preferences vary.
Best of luck!