I have some things to say-
I used to run Audioque and i was one of the first people to get their original HD3s back when they were coming off the line bad from the get go..
I was also one of the first people to get their 2200d that also came off the line bad from the get go..
I say that first to let you know how far back i go with them as a customer.
Now, i have owned both the original HDS15 and 2 HDC312s.
Both have this sound that sounds like the sub is ALWAYS past xmax.. like the sound is never linear..
I can attest to that by simply swapping them out with... well anything, lol.
Anything sounded better than what they sound like.
Now, onto box size..
Papermaker says he calculated 5.1cuft for 2 12s?
I do not know who told the OP to build that big but Soundqubed subs will FAIL in Frequency Response if built large like that!
Putting them in boxes like that will only give them a usable passband of like 10-15hz, i'm not kidding..
So.. to get a better passband.. for example, i had my 2 hdc312s in 3.8cuft NET tuned to 36hz, it did well from 32-55hz.