I think a lot of folks make the mistake and assume that all rooms sound the same when it comes to subs. The more subs in the room, IME the easier it is to overload the room....and this really needs to be taken into consideration. Having said that, it is also true that two subs can flatten out the bass response in a room...assuming that the subs are a suitable size for the room.
Since the OP has limited options of placement in his room, I would think that if he is willing to add room acoustic treatments and is considering the high level connection on the REL T9i that he will be on the right track, imho.
Since the OP has limited options of placement in his room, I would think that if he is willing to add room acoustic treatments and is considering the high level connection on the REL T9i that he will be on the right track, imho.