When I set up a pair of Velodyne HGS-15s to augment KEF Reference 1s, I ran the usual sweeps using the SMS-1 bass manager and was surprised in-room response of the KEFs was pretty flat to 30 Hz, then dropped sharply. So I low-passed at 45 Hz, and it sounded pretty good. I decided to try 80 Hz, and that transformed the listening experience, big detailed soundstage with no awareness of the subs. Marchand provides balanced passive high-pass at 80 Hz -- Charley Hanson had admonished me not to put active high-pass between an Ayre preamp and amp.
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.