Listed specs:
f(s) 37.68 Hz
R(e) 7.06 Ohms
Z(max) 76.00 Ohms
Q(ms) 5.231
Q(es) 0.536
Q(ts) 0.486
V(as) 9.340 liters
(0.330 cubic feet)
L(e) 8.74 mH
n(0) 0.09 %
SPL 81.59 1W/1m
M(ms) 282.10 grams
C(ms) 0.06 mm/N
BL 29.67
From the listed specs, I would personally enclose them in .9-1.2 cubes per @ 30-34Hz, his recommendations are definitely more spl based than freq response based, 1.5@35 would be very peaky and not get very low. 31Hz is about the last frequencies the box would make ample pressure at.
We used to use the old sdc2.5 (145.2@34 on 35amps) in out mod 1 car and I ran an hdc315A D2 153.2 @ 44 on 60 amps in my mod 3 car Would do 144.4@24 too (all pressures are headrest term lab numbers) so I do have personal experience with this company as well as building for others that I have built for.