The instructions are generic, not based on your room. Experiment and adjust it how it sounds best to you. Most subs are too loud, and crossed over too high.
Rel T/7x sub sound improvement but…
I just installed a single Rel sub to supplement my b&w 805 d4, and the sound was improved quite a lot. However I follow Rel official instructions and place the sub behind the speaker (right), and now I feel like many of the bass dominant instruments location are shifting to the right side. Is it my illusion, or is it real distortion? I don’t want to buy 2 sub to pair!