Whatever you like. In all honesty what seems to matter most is numbers not quality, not power, not any of the usual stuff everyone says. It is of course better to have four really good subs than four puny weak ones. But four puny weak ones beats one of just about anything. Something like that. Hard to explain until you do it. The minute you experience four or five decent subs in the room, well you won't believe it. The bass is of a quality so much better than you can get with any one or two subs its really hard to explain. Because whatever I say it will be interpreted in the framework you're familiar with, two speakers and one sub, maybe two, and its just not the same.
Now given your particular situation that might not be practical. Even though the additional subs could be fairly small and yet still work quite well, maybe some small Hsu sub that fits behind a sofa, you might really be stuck with just the front corners.
If that's the case then simple, get two of the Tekton subs. These monsters use four drivers and stand tall. Because of the way they are made each one is kind of like having two subs spaced 4 feet apart. Or whatever. You get the idea. DBA works best when the subs are spaced far apart and asymmetrically around the room. The idea being to create more small modes in spread out locations. Well the Tekton sort of does this by the way its made. Plus it is a powerful, efficient, killer sub. Resist the urge to place them symmetrically or in relationship to the mains. Low bass doesn't work that way. Look at my system. They are asymmetrical, no two the same distance from a corner, and yet the bass is wonderfully seamlessly integrated.
Then I would get the Dayton SA1000 to drive them. Perfect amp for this, and you can add more if you want without needing another amp. Yeah I have two but check Tim noble100 he has almost identical setup with one Dayton and loves it to death.