Are you looking for a DIY solution or prebuilt? The dual 18" PSA V3611 is close in price but absolutely destroys the little 15" SVS, it is not even close.
For half the price of both you could build the ported horn Devastator with a B&C 21" and a NX3000D.
Theater: JVC RS540U, 2.35:1 142" screen, Onkyo RZ1100, Panasonic UB820, Outlaw Audio M2200 Monoblock x3, Klipsch RF7II, RC64II, RS62II, 5800CII x4 Atmos, PSA V3611 Subwoofer x2.
2 Channel: Parasound P6, Parasound A23+, Rega Planar 6 with Ortofon 2M Bronze, Klispch La Scala II, Behringer NX6000D, B&C 21DS115-4 powered Devastator V1 x2.