A couple things to consider...
I know I have already warned you about the size and weight of the PB16-Ultra, but something else to consider is: will the box fit through your door? Measure your doorway width and make sure the box will fit through it. Also consider the sub's depth: if there are any turns in your stairwell, it might have a very tough time making those turns. Make sure you have a way to carry it up there, you will need a friend who is in good shape, and that is assuming you are using something like a shoulder dolly (this sub can not be carried up a flight of stairs by hand, even with two strong people). You really have to plan out the logistics of getting this sub into place. I did, and it was still a tough task getting it in and out of place.
Something else to consider is that the PB16-Ultra is very powerful from 16 Hz to 40 Hz, but it pays for that power at midbass frequencies. I am guessing that two PB-3000s might equal it in the 16 to 40 Hz range, and beat it above that range- and two PB-3000s would be a LOT easier to handle. And, with some care, can give you a much smoother frequency response. Two also greatly reduces localization.
Something to consider is is efficiency. The more volumetric area you allow your subs to occupy, the more efficient the system is. This should be given close consideration if you are running all of your equipment off of a single 15 amp circuit. The PB16-Ultra can pound alright, but it could also dim my lights. Multiple subs can greatly reduce the power usage and increase efficiency over the equivalent output of a single sub. Compared to the same output of a single sub, dual subs is like increasing the voltage by 3 dB but that nets you a 6 dB increase in acoustic output: it doubles your efficiency. Now think about how the equal output of one sub compares with the same output of four of the same subs! Huge gains in efficiency. Here is a system to consider: four Hsu VTF-3 MK5s. it nearly equals the performance of a PB16-Ultra from 16 to 40 Hz, but will substantially outperform it at 50 Hz and above. It would cost a bit less and eat up less power. Also consider four PC-2000s, that has similar benefits and doesn't eat up much floor space. A four sub spread would have zero localization, and should provide a very nice response even without equalization.