Pearl, glad I could be of some help. Now to some of your questions:
Yes, the SVS PB12-NSD is that much of a "beast". By that, I mean especially with home theater applications. Not to mention, SVS's legendary customer support. About the best I have ever seen from any company period! Moreover, the PB12-NSD has also been improved musically from what I garner since doing my reviews. That should tell you something about SVS's commitment to being among the elite.
As far as needing (2) subs is a question that SVS can best answer. My room was around 3,000 ft^3 when I did my review. As such, I really needed a second sub. Make no mistake about it, down low the PB12-NSD is at its best AND does so with authority. As others here have already said, I can easily verify that the SVS PB12-NSD will walk all over that Polk sub. In fact, it will do so w/o ever even breaking a sweat so to speak.
In closing, the Outlaw subs have and will continue to perform quite well with the likes of SVS, Hsu, and etc. All offer great subs in their price offerings. But like all things in life, there are trade-offs. That is why I wrote my review. To help people just like you make a well informed decision. Hence, I try to offer another piece to the puzzle. I salute you for doing your homework. Given what you are considering, I doubt you could go wrong either way. The Hsu VTF3 MKIII is the best sub I have owned to date. But, it barely beat the SVS PB12-NSD and my dual Outlaws. One day, however, I hope to own an SVS PB13 Ultra. Now were talking! Good luck in your decision.
Cheers,
Phil