Rel T/7x sub sound improvement but…

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.

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!

What are your room dimensions ?

I have (2) T5-X’s and they blend well, just takes a few shots.
I 1st will run test tones to really hear where my mains roll off to get an idea of where I’ll set the sub’s crossover…. but I do not set the gain or crossover yet.

2nd, which I feel is most important, I run just the mains with the sub crossover and gain turned all the way down and slightly increase the gain so as to match the Volume level of the mains… Do Not Want The Sub To Out Shine The Mains !

Once I get that and know how loud I want the sub to be, I do a balancing act of back and forth of Gain To Crossover to get a nice blend, generally every click up on crossover to one click down on volume and vise versa till I achieve a blend… Make sure you use a track you’re very familiar with and be patient. Also as noted in the comments above, corner placement will totally increase bass output, so placement is key, esp with (1) sub.
In my settings I tend to have the Gain lower than the Crossover.

I once read Add the Length & Width of your room & then Double it and that should be your crossover setting. IE: 12+14=26 so set your Crossover to 52…Of course I’d still play with the Gain as to not overpower the mains, but at least it’s a starting point. My Room is 12x12=24 so would be 48.
On my T5-X’s, each click on the Crossover is equivalent to 2.25hz and has a starting point of 30hz, so you do the math as to the clicks you apply. After all my tweaking I came up with 47.75hz as my crossover point, so this theory I guess was not far off.

Anyway, Most important no matter what anyone tells you…TRUST YOUR EARS !

It’s just how I do it and hope it helps.

I recently added a sub to my stand mount Reference 3A de Capo i which are spec'd down to 44 hz.  I dont have room to put the sub in a corner so i set it just behind and in the middle of the two speakers and set it for 80hz and very low volume.  Made a big difference just filled out the bottom of the sound.  not thump thump here just a fuller richer sound but subtle

I could always tell where the single sub was located even when the crossover was set low. Drove me nuts. I ended up setting up two subs using high level inputs in stereo. Both where set symmetrically with close proximity to the main speakers.  It was the only way I could get rid of the annoying imbalance.