Your budget and your statement about playing at Reference Level in a 4300 ft3 room to me are the two controlling factors in the decision making process.
The PB2+ or dual 25-31PC+ (especially since they both have a native 25 Hz tune) will simply play louder than a single CS-Ultra or PC-Ultra (which both have a native 20 Hz tune). True Reference Level (120+ dB bass peaks at the listening position) in that size room really can't be done with a single 12" driver; even the vaunted TV-12.
Personally, if I was considering the Ultra cylinder, it would definitely be the PC-Ultra over the CS-Ultra. The PC-Ultra has the tailor made on-board amp with infrasonic filters, custom EQ curves, CV 0-180 phase control, and PEQ. This feature set cannot even be duplicated with the SVS-01 Marchand Bass Box SVS offers for the passive cylinders, much less by the S1000 alone.
If you want to build for the future, you could buy a single PC-Ultra now (which will easily handle the -20RL to -15RL you mentioned the wife prefers), and then buy a second PC-Ultra down the road so you can really let it rip when the wife is out shopping.
