Wrong 3.75 as listed by DC Audio.
I believe that you missed the part that he is running 1 DC 12k amp bridged at .5 ohm to 2 Level 5s. He has the potential of haveing 4-5k per sub. Now I think that I might know a little bit about the entire line of DC subs and amps, and particularly this setup since I have been working with him to get this setup for over a year.
I recommend 3ft3 net per sub becasue of the potential of the extra power. Now if you know more about these subs/amp and his setup than I do, please indulge me with your vast amout of knowledge.
Yes you are right I did miss the amp infos..the 3.75 applies for around rated power.
Unhappily I couldn't find any T/S parameters for this sub so no way to take a look at an excursion plot for hints.. how do you know 3 cubic feet is just the number?