90 dB efficienty is maybe a little low for an OB used in the bass ? But if true the Fs is impressive: 27 Hz and the price is staying nice !
Despite the stiffness of the suround/spider, the motor seems weak (but 800W peaks allowed). The PA 18 should be better with 94 dB and more Sd, just two cents, don't know where you want to go exactly.
Imagining the Xmax is not more than 8 mm for instance, it's maybe short for EQ or very wild bafle could be needed... (or low spl programms)
I don't know if that helps: Had you already a look at the OBL15 project here : OBL-15 ? some nice subwoof reference as well (with measurements)
I just have been finding that : Base de données : Catalogue haut-parleurs
15" & 18" : look at the Xmax ! Very low ! The 18" sems a better choice and just few money more ! 250 Hz is possible as well... 24 ohms is enough if 3 in serie? (total 84 dB sensivity into 24 ohms!* if you choose the 93/2.83v - 18") hope your OTL to be a monster in power!
if you makes a U shape bass bafle with one driver per side, bafle length will stay short and you should have the floor level help (but a little loss due to the U-shape, but also a better waf !)
*: in serie efficienty decreases from 3 dB per driver added ?!
The Delta 15 B has limited excursion, datasheet says 1.6 mm.
About the Hammond, without any specs, is difficult to evaluate.