Welcome Alberto,
John is absolutely right. It sounds like a room / crossover issue. While most setup & recommendations focus on maximizing a subs output, you should really focus on minimizing the nodes. Even moving it a few inches one way or another will change its performance. Also, do a search for "sub crawl".
On a side note, be careful! I can't beleive you have THAT system in an apartment - in Texas no less!! I've heard tell Texan's get a little ornery with outsiders that irritate. And every single one of 'em has a gun! LOL!! JK!
