I read an article recently that said masonry is a terrific sound insulator for subwoofers. So if your floors/walls are concrete, it may not be as bad as you think. The article also mentioned that subs that fire forward are more neighbor friendly (to those below you) than those that fire down.
Have you seen the Earthquake supernova 10"? Also, I've heard good things about the ACI Force.
Have you seen the Earthquake supernova 10"? Also, I've heard good things about the ACI Force.