Mirko, I remember when we visited the music shop. One of the things I did when going into the various listening rooms is I clapped my hands in each room. What I noticed is that the sound dampened right away. No echo in the rooms. At home in my upstairs listening room, the exact same thing happens where it dampens right away. In your room though there’s an immediate echo if you clap your hands. My upstairs has slanted walls everywhere and acoustically I like it way more compared to my downstairs rectangle room. No contest.
Right now I’m using just one REL T5i between the speakers under the TV stand where it helps increase the bass but only because I’m just playing jazz about 65db average at night when everyone’s trying to do other things in the house. The other one is in the other listening room. When my wife is gone though, I definitely prefer both T5i’s one behind each Heresy II when playing full orchestra or some rock. Behind each speaker, the walls are angled so the bass does not reverberate off the walls as much but more so the floor. This wall or lack of wall which is slanted is why I hardly heard a difference in bass between the Heresy II and IV unless turned up louder just on their own and obviously they don’t fire down.
But when I use both T5i’s firing down upstairs behind each Heresy, the music is absolutely breathtaking! Fast, tight, in phase and perfectly controlled. Two in my room is definitely better but in your room I really think you need some room treatment because that one right corner has too much echo or reverberating and the left has much less. If you dampen the right side of the room more, I really believe you will appreciate having both subs. Try not to let the second REL go till we experiment more. I believe the real issue is the acoustics of the room, not the subs. It absolutely amazes me how the acoustics of a room can affect sound so dramatically.
It’s just like telescopes. Play all the aperture and quality of optics you want but unless you have a good environment to observe or image from, there’s a point of diminishing returns. And btw you’re definitely a better audio listener than I am and I can only go based on what these experience have shown me. Let’s experiment.