how did you stack the 2x18" ? and where's the port?
Just because I simmed 2 b&c sub18 stacked in many different ways, and I get different results. In the worst situation a 2x18" could be beaten (or equal smiley2 ) by a single 1x18" miniscoop both loaded with the same driver and same voltage applied. However these are just my sim...spl measurement give the real answer.
here's my attempt same driver loaded same voltage applied 2pi placement 2,83v.
[attachment=0:9w6fy866]<!-- ia0 -->becsub218vsmini18.jpg<!-- ia0 -->[/attachment:9w6fy866]
b&c sub 18 was 2 singles stacked one on top another with port on top. So there are one port in the middle of them and another on top. Mini is a mine design with a negative expansion and have exactly the same external dimension of 2 b&c sub18, I take care of driver distance from ground.