Okay, so I have a nice Aperion system and it sounds great.  Unfortunately, my neighbor knocked on my door tonight to complain about the loud bass noise that he hears in his apartment.

 Does anyone have any recommendations about what I can do to use my a/v system without bugging the neighbors?

Well, one thing that is happening is that sound is being resonated through the walls.  The walls will end up acting as speakers on your neighbor's walls.  While, this won't get rid of all of the sound your neighbors will hear, it will at least get rid of most of the vibrations in the floor and walls, thus reducing the bass for your neighbors.  Plus, it will help make your bass sound better and tighter.  What am I talking about?  I'm talking about the Auralex Subdude.  Do a search for it on Amazon and you should be able to find it.  Only costs $50.  Do a search on Google for reviews for this thing and you will find a lot of people who swear by it.  Hope that helps.