I'd do 1.25-1.5 with 14-16 in^2 port area per cube, tuned 32-35 hz. I did a hds3 2 12's box to 32 hz, sounds good. I just did a hds3 12 locally, let you know how that sounds when it gets built. Used to be an american bass dealer, the HD and HDC3's used to be the EXACT SAME WOOFER, I've tuned those as low as 24 hz and it was ridiculous. I have another in my vehicle at 28 hz, hdc310, great on lows, actually beat out a DD 3510 in the same box. The box is just a tool to manipulate how a subwoofer sounds. The BOX in which people limit their thinking is not really there.
---------- Post added at 08:15 PM ---------- Previous post was at 08:14 PM ----------
34 and up gets more high bass then I like. 35 hz usually reaches up fairly high with those woofers, and it's strong too. 40-60 hz is strong.