This misses the point of this system, it is the same size or smaller than virtually all dual 18 inch subwoofers, offers lower distortion and less power compression AS COMPARED to dual driver systems.
Yes it is true when looked at as a quad driver system it would be 3 dB louder if it was not an isobaric design it would also have to be 3dB bigger.
And as for a horn loaded alternative it would have to be much larger to go anywhere near as low as a vented system, but you do get the efficiency.
Kenneth,
Since you don't post any frequency response curves, hard to compare sensitivity or efficiency of the AS-418 to anything else.
The EAW SB-1000/1000Z/1000ZP have half space sensitivities from 99-102 with similar LF claims as the AS-418 in a slightly smaller cabinet, 15.64 cubic feet compared to the AS-418 at 16.54 cubic feet.
An SB-1000z was compared to the smaller Danley TH-118 tapped horn (14.41 cubic feet):
http://soundforums.net/varsity/6793-th118vssb1000z.htmlThe slightly smaller single driver tapped horn had slightly more upper output, the SB-1000 slightly more extended LF.
The B&C 18SW115-4 used in the DSL TH-118 has more Xmax than the drivers used in the AS-418 (look like a shorter winding version of the B&C18TBX100), my money would be on the TH-118 having higher ultimate output.
As far as the reduction of even order distortion (leaving the odd sounding odd order distortion) the push pull isobaric arrangement provides, that would be something to consider, though it seems few care about even order distortion, as it makes a sub "sound louder".
Art