try turning down the base on your Subwoofer they are designed to convey the lower bass tones of the home theater experience. Subwoofers increase the quality of both movies and music by providing richer, deeper sound quality while enjoying your home theater. Combined with a surround sound package, subwoofers bass radiates throught out the house and is possible bleeding (humming) through the walls or the floor, when I watching a movie in my living room watching a movie and I go into the kitchen to get a drink or whatever, I can here the bass, almost sounds like thunder I have a pretty big house! so Maybe if you turn down the subwoofer you might not have any more complaints
all the surrond systems have a tendency to give you better quality sound, better than hearing what the heck they are saying? so I find myself constantly turing up the volume to hear what is going on! until i figured out to turn down the subwoofer, although if your subwoofer is not independant of your system there isnt too much you can do but move it away and off the floor,this would help muffle the bass. it would help being off the floor has the bass will go through the floor. good luck and I hope this works if not try inviting your neighbor to a movie at your place! maybe they would enjoy your system they might just want to get their own?