Due to my room and cabinet I am restricted to using bookshelf speakers with 8 inch drivers.
Adding a sub makes a more full sound. You also muddy the midrange if you are asking your speakers to play a 50hz tone along with a 1000hz tone
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.