@Tweeter_Box: With the magnet inside the chamber I would say 35 - 40 Hz, 12 - 24 dB/ octave. What sounds best, is best untill proven otherwise. To your previous 2 questions I believe the answer were yes and yes, can't remember exactly what questions they are tho.
@Crossed: that's good news, you might be able to use 6 Cubo's for two 2 x 12" tops (/ 1.5") if it's bass-heavy music. The again, if I had your current setup I wouldn't need my system to grow
@PrettyHard: It would work but I don't think it would give the deepest bass possible and need a bit of power to get going Xmax-wise everything is very OK. Someone from the Philipines has build two and since he responded quite happy I asume it went well.
I would start with Cubo 18 Extended, with the magnet outside. I like the LF18G401 as it's pretty efficient (3% +). If you want to maximise on efficiency you can put it with the magnet inside the chamber but with the magnet outside would be my initial bet.
thanks a lot , i´ll start with 4 pcs in next month i hope ! and when they´ll be completed and tested , i´ll give some feedback ! good luck and i am sorry for bad english i am from slovakia as i said before .