You obviously are not familiar with Cornwalls which are capable of prodigious output. They will swamp two 12" drivers. I many systems subwoofers do more harm than good. If the OP is a critical listener and continues along the path he is on he will be very disappointed.
@mijostyn First, the OP has already said he’s not satisfied with the bass he’s getting and is looking for improvement, so clearly their bass output is not all that prodigious in his room. Second, there’s no way a pair of 12” subs that go down to 20Hz -3dB will not add meaningful information that the Cornwalls are simply not physically capable of — that the subs’ drivers are not 15” is determinative of nothing. And third, he can try the SVS subs with absolutely no risk, and I’d bet big $$$ he would not be disappointed and would not return them. There are precious few speakers (if any) that wouldn’t get at least some benefit from adding two good 12” subs just to better manage in-room bass issues if nothing else, and the Cornwalls are not “magic speakers” that somehow defy physics and logic.