I think of you are lucky enough to get your nearfields to go down far enough then the need for the sub is less somewhat. I use Mackie HR 824's and before they were on a solid wood structure that caused it and other things to vibrate.
(Put 40 Hz tone into your room and feel everything around and see what is moving) This caused some interest previously but I put the Mackies on very heavy concrete stands and anything that was vibrating before stopped and the bottom end of those speakers changes a lot. Less colouration and extended.
I have just done a test and the Mackies in my setup here are only -3db down at 32 Hz so that is not bad. Their spec says -3db at 37 Hz so it is well down compared to that frequency. At 30 Hz though they are -6db so they are dropping off. I have never had a problem mixing without a sub and are one of those that can. The car tells me a lot actually about what is really going on down there.
If I was to add a sub in then you have to consider the crossover frequency. I have two options and that is to go from maybe 20Hz only up to 35Hz or so and let the Mackies take over from there.
(as they do naturally) Or crossover to say 90Hz and let the sub go higher and ease the low end away from the Mackies, also a good option. They will sound different though because different speakers now are handling say 40Hz.
Our old friend Bob Katz is also a bit of an expert on subwoofers and if you are after interesting reading try here:
http://www.digido.com/articles-demos.html Scroll down to the article near the bottom of the list No 19 Subwoofers and check it out.
Read all the others on the way down too!