Actually, doubling the cone area (adding a second sub) should raise
the output by 3dB, even without increasing amplifier power. Adding a
second sub, and giving it as much power as the first sub will give
approximately a 6dB gain, not 3dB.
So to use your example, two subs getting 75 Watts each should be 3dB
louder than a single sub receiving 150 Watts, assuming the subs are
the same type.

What I always believed(and proved in my college physics class) was that if you
have one object at x decibels, and you put another one near it that is x
decibels, the sum is x+3 decibels. In other words, if you have a race car at
130 decibels, and you put another race car next to it at 130 decibels, now you
have 133 decibels. In other words, if you double the intensity I, you have a 3
decibel gain.
So that should work for subwoofers the same way, so adding a second subwoofer
powering it exactly the same as the first will yield a 3db gain.

To quote from a physics book:

"SL = 10 log I/Io (Io = I sub zero)
The SL is definined with respect to a reference intensity Io(I zero), which is
chosen to be 10^-12 W/m^2 (a typical value for the threshold of human hearing).
Sound levels defined in this way are measured in units of decibels(dB). A sound
of intensity Io has a sound level of 0dB, wheras sound at the upper range of
human hearing, called the threshold of pain, has an intensity of 1 W/m^2 and a
SL of 120 dB. Multiplication of the intensity I by a factor of 10 corresponds
to adding 10dB to the SL. We can also use dB as a relative measure to compare
different sounds with one another, rather than with the reference intensity.
Suppose we wish to compare two sounds of intensities I1 and I2:
SL1 - SL2 = 10 log I1/Io - 10 log I2/Io
which equals 10 log I1/I2"

So, if we double the intensity(by adding another subwoofer with the same exact
wattage) this yields and intensity ratio of 2. So we have 10 log 2 which equals
3.01, or, a 3dB gain by adding a second subwoofer powered the exact same.

The book goes on to say:
"For example, two sounds whose intesity ratio is 2 differ in SL by 10 log 2 = 3
dB" which is what I just said.

Jamie