Look at the Arendal 1723 1S subs. They have a very gradual roll-off and will net you a deeper response than the ULS-15 subs. You could get three or four of them, and then calibrate with Dirac Bass Control, and, if they are placed well, you would have just about perfect bass to well below 20Hz. Alternatively, if you have room, look at the Hsu VTF-15h mkII, they can be tuned to 16Hz, and two or three will get you bowel-churning deep bass as well as a perfect frequency response with good placement and calibration. Same thing with the Monolith subs, especially the 15".