If you're trying to tame the sub, auralex foam won't do much -- foam will absorb high frequencies and that sub crossover is probably below 150 hz. You can do some diy bass traps, look up ethan winer posts, he's a bass trap nut. Tri corners (wall/ceiling corner of room) is biggest priority. I've just installed some chunk traps, in my theater build, though it's not going to be justified in a rental.
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If you are super dedicated, you can hunt down some HVAC fiberglass board -- vs. buying 703. The HVAC stuff is identical, more readily distributed and cheaper. Then cut it into triangles (2x4 panel can yield 8 pieces), but you will need a ton of time, plenty of showers (the stuff is a PITA to work w/ and itches!)...
The cheapest and first thing I'd do is simply move the sub around. Sometimes pointing it into a corner or moving it 10 inches will eliminate a null at your seating position. If you want to reduce the boominess outside of the room you are listening in, that's a whole other issue from sound optimization inside the room. Unfortunately, w/an open space you're SOL.