Rel T/7x sub sound improvement but…

Have you tried lowering the crossover frequency?  And maybe turn the gain down a bit?

Is the sub in a corner where you might be getting a lot bass reinforcement?

Feel free to move the sub around to get the sound you want. The REL instructions are a suggested starting point. I know you stated you don’t want a second sub but I would highly recommend one. I had one previously and upgraded to two and it reduces the kinds of issues you are hearing as well as bass nodes. It increases sound stage and imaging as well. IMHO it’s well worth the cost.  By the way, nice subs I have two T/5x subs and they are great.

@sls883 brings up a good point as well, if the sub stands out you’ve likely got some integration to work on with crossover, volume and phase as well as placement.

I tried to lower the crossover frequency so mainly output the non-directional bass, but that suppose will break the smooth crossover from main speaker to sub. I really do not have enough space for another sub, so maybe I just place the sub at the middle of the main? (I did not see anybody doing that in yt)

When I only had one sub, I placed it just left of the right speaker and it worked fine. To me, it makes sense to put the sub between the two mains especially if you have to set the crossover frequency kinda high. But, I'm no expert. 

I too have a single REL T/7x that I am using with a pair of stand mounted Tonian Oriaco speakers.  I found that corner placement worked the best just as REL suggests.  The sub is to the left of my left-most speaker.  

If you have a sub adjusted properly you shouldn’t “hear” it at all.  You should simply hear more weight and lower overall extension.  I use the warble tones from the Stereophile Test CD and a SPL app on my phone positioned at my most common listening position to find the smoothest response and best extension, but there are many ways to set-up a sub.  

I enjoy the way the single REL T-7x enhances my listening.  I hope you will too once you get it properly integrated for your room and speakers.

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.

Sounds like you have possibly both the crossover and volume on the sub set too high.  If you want to get better results it takes a little work upfront but well worth it, and once it’s done you’re done.  First, you want to find the place in your room where the bass sounds best and then fine tune the sub’s settings from that position.  This first link shows the proven “crawl method” to find the best locations for the sub for bass in your particular room, and the second link is a simple guide to effectively set the sub’s controls to optimize performance. 

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

https://lifehacker.com/how-to-properly-set-your-subwoofers-volume-without-sh-1506136549

If you can’t do the crawl method initially, then Todd Welti who’s an expert in all things subs recommends placing the sub 20% into the room from the front wall and 25% into the room from the side wall, so you might try that placement and see what you get until you can do the crawl.  Hope this helps, and best of luck.