Jamie (Bandit) is claiming that doubling the cone area AND doubling
the power delivered will only give a 3 dB increase, which is simply
wrong. He would be correct if we were talking about point-source
energy sources, like his textbook describes, but with loudspeakers at
low frequencies, cone area has an effect as well as total power. Note
that this phenomenon does not apply to tweeters. This is because the
speakers must be mounted close together (relative to the wavelength
being produced) for the synergistic effect to take place. Subs are
dealing with very long wavelengths, so this is easy to do. Tweeters
produce energy with such a short wavelength that even if you put them
side-by-side, you probably still wouldn't get the effect.

I was just going by the rule that doubling intensity gives a 3dB gain. I
figured that putting another subwoofer next to the original with the same
amount of power would serve to double the intensity. But, it seems that cone
area behaves similarly yet independently of the intensity, ie 3dB for doubling
it.