Some of us may be spoiled, lucky or otherwise gifted (through long, tedious hours of experimentation) with a subwoofer pairing that has ended up very successful. Going on to make more general statements as to their making wonders in other constellations mayn't be prudent, given that quite a few factors could as well lead to the opposite scenario. Claims are no doubt made about DBA setups being very generally worthwhile additions, and they may be so in certain respects not least including frequency response, but that hardly makes them an all-encompassing win in my book - certainly where asymmetrically placed, mono-coupled implementations are concerned. I guess I'm old schooled in that regard - if there is such a thing if only to attain specific traits and ultimate coherency in audio reproduction - maintaining the importance of a high-ish crossover to the mains (that should be high-passed) which further dictates symmetry-and-close-to-the-mains placement and stereo coupling of a pair of subs. Oh, and not least: big, efficient and preferably horn-loaded sub iterations are a must, to my ears, all of which should more easily pave the way for a successful outcome. High-passing the mains I might add would be best achieved running the whole speaker set-up actively, so that any alterations will be done pre-amplification, on signal level and without further interference of a passive crossover. Make of it what you will, but that's the context through which I find subwoofers are implemented most naturally as a seamless part of a whole.
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.