Correct, the drivers in the Submersive aren't landmark drivers by any means from what I can tell, but the nice thing is with the box design, they stay well within linear tolerances with not much need for excursion at all to produce really decent output and have little distortion if any, and the design is quite efficient running off a single EP2500.
Here is a good thread where a guy used a Pair of AE AV15-Xs and built his own
submersive clone, and from what I can see from his results - it performed very very well in a 136 L or 4.80279467 cu feet box... roughly the size of a SVS Ultra13.
$500 for drivers
$300 for amp
$100 for supplies (mdf & connectors)
EQ which you already have...