Dank,

Subs came to pass as a result of our trying to get low bass from smaller enclosures.

A woofer in a large enough cabinet would create bass that was harmonically in sync with the rest of the music due to the fact that all the frequencies came from the same point source with the same phase angle.

The only advantage to having a sub might be that you can play music louder with less distortion.... or create the illusion that bass is coming from small speakers elsewhere in the room... or simply use the fact that you have two amplifiers as a tone control by turning up or down the bass independently from the main speakers.  Of course this can accomplished with 3-way loudspeakers that have been bi-amped or even tri-amped so far as the amps match.  Remember every amplifier has a slightly different phase angle, just like speakers.

So, the only advantage a sub offers is lower distortions at high SPL's and smaller box sizes.