I am also making two 18" subs, for my Magnepan 20.1 extension with a very low xo of 40 or even 30 Hz.
They will be U-shape, with small but evident different shape to avoid cavity noise .
I will use the FI IB318(very similar to AE but even higher linear X max) drivers arriving in two days directly from US.
What do you think, could it work well?
here the specs:
SINGLE 2 | SINGLE 4
Fs: 22.3 Hz | 22.5 Hz
Re: 1.5 Ohms/coil | 2.8 Ohms/coil
Qms: 3.33 | 3.41
Qes: ..81 | .77
Qts: .65 | .62
Mms: 272g | 270g
Sd: 1210cm^2 | 1210cm^2
Vas: 381 l | 381 l
Spl: 89.1dB 1W/1m | 89.4dB 1W/1m
Bl: 8.43 N/A | 11.8 N/A
Rms: 550W @ 20Hz