Going full range "may not get a perfectly smooth sound" either, especially if you're trying to get that balance of smooth extended bass *and* optimum dispersion, soundstage, and imaging.
01-15-14: Kennythekey
I agree with Karl_desch, so upgrade your speakers if you must improve your bass. There's a reason you must spend considerable bucks for a true full range. As Karl put it, "coherent balanced sound." Adding a sub may give you the low frequency you crave, but you may not get a perfectly smooth sound.
At least with a sub you can do several things about integrating the bass with optimally placed monitors--change the subs' gain level, change the crossover frequency, high-pass the mains or not, change the phase, and change the location. Sure, that's a lot of potential adjusting, but it also allows several bass integration options that are not possible with full-range speakers.