I have a DD-15 and it is very powerful. I tried the DD-12 and it was very impressive as well. I went with the 15 just in case. Everyone that listens says it's the best sub they have ever heard. Even my friend that has two Aerial subs and a DD-12 for his low frequency effects says it's the best.
There are also people that claim bass can be cancelled with two subs because some bass is recorded out of phase. I think there is always a trade-off. If I were going to use two subs, I would go with the DD-12s.
I don't believe in the speed issue because to produce the same amount of bass a smaller sub has to move further. Therefore it cannot be faster unless it produces less bass. It could actually be slower when producing the same bass as a larger sub. A smaller sub also has to work harder. The only real advantage I see in size is a smaller footprint and less cost.
There are also people that claim bass can be cancelled with two subs because some bass is recorded out of phase. I think there is always a trade-off. If I were going to use two subs, I would go with the DD-12s.
I don't believe in the speed issue because to produce the same amount of bass a smaller sub has to move further. Therefore it cannot be faster unless it produces less bass. It could actually be slower when producing the same bass as a larger sub. A smaller sub also has to work harder. The only real advantage I see in size is a smaller footprint and less cost.