It's just that a subwoofer that's tuned for lower frequency extension will go deeper, that's all. The three π will just hit 30Hz, but a dedicated sub is usually going strong at that frequency and will go all the way down to 20Hz or below.

If you want the real deep bass at earthshaking levels, like for home theater, then adding subs is an easy way to do it. They're also very useful for smoothing room modes in problem rooms with no damping. In that case, you're not looing for additional output at all, you're just filling in holes caused by exaggerated room modes. Either way, the subs are fairly easy upgrades, if wanted or needed.