It does make a big difference, but so does a 10th of a second in an olympic race, or the length of a horses nose at Belmont. The speakers in the SX sound fantastic on their own, (I used them alone at the winery yesterday, and it was awesome) but for a little extra "ooomph" there's nothing like adding that big fat bottom.
In a studio, or home setting They are more than adequate, and actually quite lovely. It depends on what you're going for ... sound-wise. For home, with no dancers, and not a lot of real estate to cover - the bass response is well balanced, but if you need to move some air for dramatic effect, or you're playing lots of jazz bass tones, simply turning up the volume doesn't match the efficiency of nice beefy sub.
*** And, for all you New Yorkers ... I'm not talking about HOAGIES, either.