You have obviously done quite a bit of research, and have given this a lot of thought. But, there is something that you are missing. The graphs that you posted are both taken from the SVS website. They show the basic shape
of the frequency response, and that is very helpful. But, what they don't show is the maximum output of the subwoofers, and that can also be important when you are comparing specific models and trying to determine how much output will be enough for you.
For instance, compare the measured output difference between the SB2000 and the PB2000, in the same review you referenced:
If you compare those numbers, you will see that the PB2000 has slightly more max output, at every frequency from 63Hz down, than the SB2000 has. Below 40Hz, that output difference becomes profound! Although there aren't any comparable measurements which would allow us to see max output numbers for the PB1000 and the SB1000, the output difference between the two, below about 40 or 50Hz, would be similar.
That is a fundamental characteristic of ported subs. They typically have more output, below about 50Hz, than comparable sealed subs, due to the larger cabinet volume and the ports which also move air to create bass sounds. At low-frequencies, room gain can help to make-up some of the low-frequency difference between sealed and ported subs. (Both benefit in the same way from room gain, but ported subs roll-off much faster below their port tunes.)
My guess is that sealed SB2000's will give you just what you are looking for in a 1400^3 room, because you should be getting a lot
of room gain below 20Hz. But, that may depend somewhat on your overall listening level, and on how much additional bass you like to add to your overall volume level.
Ported subs also give you much more tactile response (TR) for low-frequencies--deep bass vibrations you can feel. That is due to the ports themselves, which move air, creating additional particle velocity. That can be a very good thing for people who are on concrete floors, because concrete laid on soil is relatively inert. But, on a suspended wood floor, ported subs (especially in a small room) can actually give some people excessive tactile ULF, because the floor itself resonates with <20Hz frequencies.
So, even in a very small room, some of this depends on your room construction, and some of it depends on your own listening habits and preferences. I think that for most people, dual SB2000's should be plenty! And, that's probably what I would recommend in the majority of cases.
But, if you are on concrete, and if you really want a lot
of low-bass SPL and TR, I would probably try to go with either the dual PB1000's or even the PB2000's. For many people, dual PB2000's would be way more than they need in a room that size. But, I have seen people who enjoy even more powerful ported subs than those in rooms that size.
I wish that I could be more helpful. But, if this were an easy question to answer, just based on room size, there wouldn't be so many threads like this one.