Two PC12 NSD for 950 has to be the best bang for the buck at the moment. I would grab them while the getting is good. There are better subwoofers out there, but not for that price, as far a I know. Not even two will come close to a Rythmik FV15HP in output and extension though, but a FV15HP costs a lot more. On the other hand, with careful placement, two PC12 NSDs should be able to do more to smooth out the frequency response, and two will take greater advantage of Audyssey's SubEQ which is onboard your Denon X4000.