With my hidden enclosure I have 8.7^3, under the top enclosure will sit a enclosure in the spare tire well that is 2.5^3 exactly. So I have a total of 8.7^3 to work with.
And also IMHO DD is not the best subwoofer out there, I have come to see that there are alot more companies out there that get louder and deeper than DD. No their amps on the other hand, I love em hands down, but the subs......Imma have to disagree with you on that.