I had a set of Spendor sp 1/2e's and a James loudspeaker emb-1000. having a sub to fill in the bottom adds so much depth and even width to the soundstage. There is a lot of information in those lower frequencies that are responsible for the real and in the room sound. I think if you are accustomed to fullrange speakers, you would have to have a sub or it would sound like something is missing. I just replaced my set-up with Vandersteen 3a's which go to around 20hz. Even though I had close to that with the monitors and sub, the bass is much more seamless, coherent, and detailed in the Vandersteens. When I say coherent, I mean when you hear a bass string plucked, all the high frequencies of the finger hitting the string, the pluck, fretboard noise, are in the same place in the soundstage as the bass note itself, as if you were in front of the actual bass being played. Sometimes with a sub you are very aware of where the sub is, you hear the bass note coming from the sub and all the other surrounding information is coming from another place on the soundstage.I think your success or failure will depend on your choice of monitors and sub, the rest of your system amplification, how they all mafit together, and how much time and effort you want to put into integrating your sub. Most people would recommend that if you have the funds and space, get two subs.
good luck.
good luck.