A pair of SVS-12-nsds for $1149 is hard to argue with from a value standpoint. A possible issue is they are down 3dB at 23Hz, which is only a couple Hz below where your speakers supposedly drop off (although Tyler didn't publish +/-dB figures). I'm sure they'd still make a pretty big improvement, but if you're looking to go deeper something like the SB13 Ultra might work better. If it's out of your budget I suppose you could start with one and buy another one later, although you save a couple hundred bucks by buying two but still $2999 so quite a bit pricier.
If I'm you and given your system I'm definitely doing dual subs, and if budget is a concern a pair of 12s seems very enticing (I'm considering them myself). Feedback I've gotten from people who've used them was very, very positive from performance, build quality, customer service, and value perspectives. I'm sure some of the other options listed above would be fantastic as well, but I'm not familiar with their cost but suspect they may be a bit more expensive.
Best of luck, and I'd be very interested in whatever you decide and how it works out.