Musically the important thing when adding a sub to a system is that a high pass is used on the main speakers. Then as stated distortion foes down and the main speakers open up significantly. Without the high pass this won't happen and the only advantage of a sub when integrated with the room is more bass and improved sound due to the low bass supporting the entire frequency envelope. This matters but isn't as important as taking the large load off the bottom of the main speakers.
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.