Based on endless testimonials and assurances that SVS cylinder subs and box subs are nearly interchangeable in performance (at least in current models), and on the independent measurements I've seen of the PB12-NSD (box), and on SVS's published specs and measurements, I feel pretty confident going with PC12-NSD (cylinder) in my new HT.
Having said that...I would still love to find some decent objective measurements for this model if I could before taking the plunge. The only ones I know of are the minimalist measurements from Sound & Vision:
22 to 235 Hz, ±3 dB
Low bass (40-63 Hz) average: 118.1 dB
Ultra-low bass (20-31.5 Hz) average: 115.6 dB
Which has a bit less extension than SVS states (18Hz), and less extension than the SB12-NSD measures. Still impressive numbers, but a little richer detail would be nice.