I have two Rel T5/x paired with my Dynaudio Special 40 bookshelves.
My first experience with subwoofers, I initially followed Rel's guidance and put them in the corners of the front wall.
That produced lots of bass boom, prompting an all-day exercise, moving them all around my listening space (a living room like yours) to try to get upright bass transients clean and clear.
I ended up with them out in the room, on either side of the couch where I listen.
Strongly suggest (as others have) a room mode calculator like this one to try to understand where bass is piling up: Acoustic calculator: Room modes
That and a tape measure per Hans' room correction recommendations helped show me where to cut out certain bass frequencies below 300 hz with eq, in my case using Roon's eq feature. Roon eq is really useful because from your listening chair you can turn on and off the frequency band stops you assign based on your modes and room measurements and see how the sound changes.
Free basic room correction - YouTube
Can't say I've got a lot of boom now.
I also have the gain and crossover settings, that I set by ear, quite a bit lower than Rel's recommended starting point. This is likely due to my mostly square room, which I believe really accentuates the bass for me.
I'm no expert, just my experience. Happy listening.