Hey Lampy,
John pretty much covered everything. Keep in mind the terms "small" or "large" have nothing to do with the physical size of the speaker. It is more about its ability to play bass below the frequency crossover setting efficiently.
Yes the 60's and 80's can pump out some potent bass, however, the subs larger driver can move a lot more air within a room, especially when you get down to the lower Hz. Also, anything below 80 Hz or so is non-directional, meaning you can't tell the direction the sound is coming from, you just feel it all around.
Also, keep in mind when you set a crossover to say 80 Hz, it does not just cutoff at that frequency. There is a gradual slope or blend between the speakers and sub.
It is also a THX recommendation.
