IB - Infinite Baffle Subwoofer Build Projects
There is certain order to fill specs to sheet. Qes, Qms, Fs, vas, Mms, BL, xmax and Pe. Software calculate others.
Two FI IB318 will outperform four AE by 6db, but AE's goes bit lower. (f3=17Hz vs FI 25Hz)
Shouldn't make difference in your concrete bunker, there bottom end could be too rich with...