Get a second sub, go stereo
There's plenty of cheap subs. Emotiva's got a few.
My place is a duplex with 780 sq. ft. and I'm a cyclist....bikes everywhere. I'm lucky my wife does not make me sleep with my amp under my pillow. Cramming a single sub in here is going to take some finesse.
The sub is just for fun anyway, wanting to see if I think it improves the already great sound my system is kicking out.
FYI, I went with the Behringer MX400, so with cables and adapter plugs, I'm in for another $55. We'll see how it all plays when it gets here and hooked up. I would have used a passive box, but not one of them, even the expensive ones, has a decent specs document. I figured that if I was going with the relatively unknown, I'd go with a unit that logically should not worsen the signal to the amp. With the passive units, adding crosstalk seemed to be the price I'd have to pay.