I've read that a dipole is a velocity source, not a pressure source. Does this mean the room is pressurized less with a dipole than when using a monopole source for bass? I would imagine a less pressurized room would transmit less sound to neighboring rooms, am I correct?
Sounds reasonable enough. With a dipole you're interested in the amplitude at the lobes of its radiation pattern, while a conventional loudspeaker is almost omnidirectional at low frequencies.
Unfortunately, at frequency 0 a dipole inevitably is an acoustic short (no velocity, duh), resulting in a steep (4th order) bass dropoff. You can lengthen the path from front to back by making the baffle bigger, but for a sub that's not going to be small. Unsurprisingly, dipoles tend to need a lot of power and long throw at low frequencies. So not the most practical way of going about this, unless you are willing to go with in-wall mounting (as some people have done with subwoofer arrays).
Speaking of arrays, not allowing room modes to build up by absorbing bass at the back with an appropriate delay (i.e. a DBA) would have the same effect of "depressurizing" the room. Unfortunately setting up a DBA is neither cheap nor trivial, though I imagine you could make a decent approximation of one using closed box (CB) subs in the middle of the front and back wall.
Now before you consider going to these lengths, I would recommend experimenting with what you have. #1, try placing the sub right in the center of the room, and by that I mean in all dimensions. You may find that sub levels will go down quite a bit, especially when compared to a position on the floor near the wall or even corner. Some concepts like downfire subs deliberately make use of boundary effects to boost SPL output, at the expense of exciting a lot of room modes. Not relying on these implies using a larger, more powerful sub - probably a large BR or even larger CB depending on levels required.
What kind of sub and what kind of house construction are we talking about anyway? In a massive building the most severe issues may be related to room modes, while in a lightweight construction (which drains bass like nobody's business) the best thing to do may be getting the sub closer to the listening position.