Yes 12 ohms is better. I've got two Dayton amps running 4 subs, all 8 ohm drivers. Tried all combinations, two and four per amp, 4, 8 & 16 ohms. Technically, on the face of it you would think that since these amps put out multiples more power into 4 than 16 that the lower the better. Probably if you care more about meters than music then 4 is better. More power! Listening to music however its clearly more tuneful and articulate, impressively so, at 16 compared to 4. Which corresponds with what Duke told me, that pretty much all his customers prefer that as well.
Your question of why they are made that way is complicated. Why they choose to design for a particular impedance is a whole different question than which sounds better when all you are changing is series/parallel.
The bigger question to me is why put all those in two cabinets when putting them in four would be so much better? The improvement you'll hear between how they are wired is nothing compared to how much better the same drivers will perform in four cabinets vs two.
Your question of why they are made that way is complicated. Why they choose to design for a particular impedance is a whole different question than which sounds better when all you are changing is series/parallel.
The bigger question to me is why put all those in two cabinets when putting them in four would be so much better? The improvement you'll hear between how they are wired is nothing compared to how much better the same drivers will perform in four cabinets vs two.