Well, here are some initial impressions regarding using two of the PC12-NSD subs:
Bass seems more effortless, with absolutely no sense of strain no matter how loud I play music or movies. Not that there was a problem before, but now I can clearly hear that while my front mains (JSE .6's) driven by a B&K ST-202 150wpc pwr. amp begin to sound strained at extremely high volume levels, the subs are just cruising along, pumping out prodigious amounts of bass, effortlessly. On music, anyway, the reach of these subs seems very, very good. I've yet to put on my movie blockbusters, but will report when I do.
The bass is very tight and well-defined, as well. No boominess, just gut-busting bass notes when the source material calls for it.
The main reason I went with two has to do with room size and acoustics. The room is not that big - 15'x19', but there is a 6' opening in the left wall that opens into another 12' x 19' room. I felt there was a "pressurization" problem with only one sub - that's a lot of cubic feet. AND I felt that any room node problems (and audibly, there were some) would be smoothed out by using 2 subs in 2 locations. Also I knew with two, they should be just loafing along, no matter what demanding stuff I threw at them.
When I installed them, they are replacing an almost-new DefTech SCI subwoofer, which sells for about the same price that a pair of these do. While the DT is well-made, weighs a ton, and can play pretty loud, it really doesn't go all that deep and cannot begin to really move some air in that size room(s). One PC12-NSD actually outperforms the DT (IMHO), but two of them really smoked it's performance.
I think, after making this switch, that you simply can't beat physics. These companies can make smaller and smaller subwoofers with ever more powerful amps. But you cannot beat having some *volume* (as in cubic feet) inside your cabinet if you want to get low, effortless bass reproduction. It just ain't gonna happen, in my opinion.
So I can say, after using an M&K (MX125? 2-12" subs w/400w amp) for years, and the DefTech SuperCubeI for 4 months, that the SVS subs are just the cat's meow. Finally, I've got more than enough deep, tight, effortless bass to put a firm foundation under music and movies. And to think I only found out about 'em on this forum the middle of last week!!
Thanks to all who recommended the brand....