When looking to upgrade from the "toy" monitors (you know the type with 2" speakers) I looked into several but ended up going with the Yamaha HS80ms (8")simply because I felt with the 50s (5" speakers) I would be looking for that sub very soon. With the 8's I do not feel a need for a sub at all.
As the others have posted earlier, adding a sub to your studio requires a little more thought and research than just adding some boom to your home theater set up. So do your homework and figure out if you actually need a sub BEFORE you buy.