About that claim of 136 dB SPL at 20 Hz...
Beranek's Acoustics in equation 4.17 gives a formula for the acoustic pressure of radiation from a piston on an infinite baffle into half-space. This formula can be expressed in a form such that it gives the on-axis RMS acoustic pressure as a function of frequency, surface area of the piston, peak displacement and distance. That formula can be expressed as:
p[SUB]RMS[/SUB] = sqrt(2) * pi * f[SUP]2[/SUP] * rho * S[SUB]D[/SUB] * x[SUB]p[/SUB] / r
where:
p[SUB]RMS[/SUB] = RMS acoustic pressure in Pascals
f = frequency in Hz (= 20 Hz in this case)
rho = density of air (= 1.2041 kg / m[SUP]2[/SUP] at 20 deg C)
S[SUB]D[/SUB] = surface area of driver in m[SUP]2[/SUP] (= 0.32835 m[SUP]2[/SUP] total for two 18" drivers)
x[SUB]p[/SUB] = peak displacement in meters (= .034 per Magico's 34mm claim)
r = distance in meters (assumed to be 1 meter)
To compute SPL, you calculate:
SPL = 20 * log[SUB]10[/SUB](p[SUB]RMS[/SUB] / p[SUB]REF[/SUB])
where:
p[SUB]REF[/SUB] = 2 * 10[SUP]-5[/SUP] Pascals
Plugging in the numbers above, you get SPL = 121.5 dB. That's 14.5 dB short of their 136 dB claim, which is equivalent to needing the radiating surface area S[SUB]D[/SUB] to be 5.3 times larger. Stated another way, and rounding to an integer number of drivers, it would require 11 18" drivers with 34mm x[SUB]max[/SUB] to get there.