I’ve been fighting with room acoustics and a REL T9x for over a month. I was at the point of selling it until I went completely against the set up recommendations and moved it out of the corner, raised it onto a 1” granite slab and decoupled it with Herbies grungebuster pads under the feet. Now everything clicked into place, bass grip and definition, soundstage are spectacular. best of all, the relentless room resonance is gone. I don’t like the aesthetics of having treatments all over my apartment. If I ever get a dedicated listening room I’d be all for a different approach but for what I have to work with now, a sub is a phenomenal acquisition. I’m sure others are in the same situation.
Why I'm not adding a sub to my 2-way monitors for music
I've updated my blog post with more data, more analysis and the conclusion that in my particular case I do not need a subwoofer. I encourage everyone who is on the fence about adding a subwoofer or not to read and comment. I think that either way you'll be able to make more informed buying and configuration choices.
Everyone's situation is different but I hope the post helps you get to the right conclusions.