6 dual 2s: -- wiring each sub in series to 4 ohms, then the 6 4 ohm subs in parallel to get a .66 ohm load.
-- wiring each sub in parallel to 1 ohm, then the 6 1 ohm subs in series to get a 6 ohm load.
6 dual 4s:
-- wiring each sub in series to 8 ohms, then the 6 8 ohm subs in parallel to get a 1.33 ohm load.
-- wiring each sub in parallel to 2 ohms, then the 6 2 ohm subs in series to get a 12 ohm load or in parallel to get a .33 ohm load.
R total in parallel = (1/R1 + 1/R2 + 1/Rn)^-1
R total in series = R1 + R2 + R3 + Rn
It's really that easy.