Rel T/7x sub sound improvement but…

@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.