@Urouge:
Putting your speakers on a carpet/rug would be a good start. It'll absorb a good deal of vibration without translating it to the floor, which is probably what he's hearing.
It's not usually a knob itself that controls the bass, but rather an option in which the volume knob controls the bass. Try clicking buttons like "setup" to scroll through the options. Eventually "Bass" should be displayed on the LCD.
I am trying to get one of those auralex gramma pads for vibration control. I hope that helps! 
@captain:
uhh, I HATE HATE HATE HATE HATE bass playing through walls >< ya, it doesn't matter how quiet or loud the music is. the vibrations of the base are going through the wall. Carpet sounds good to me. or anything else that's gonna be a barrier between him and the sound. Or, can you move your speakers/pc, whatever, away from his wall? Some people are extra sensitive like me. maybe it won't bother your neighbor on the other side too much.
Also, what time of day or night are you playing this stuff? if it's 3am, then ya, your being inconsiderate. You can't be expected to be quiet 24/7 though. you gotta have some fun too. I mean, what about practicing your guitar. Maybe, he works graveyard shifts, and you keeping him up during the day when he needs his sleep? Can you go talk to him and work something out, where you set a time where your gonna be noisy, that won't bother him? like when he's at work or something?
I know that its a pain in the ass to hear bass from neighbours continuously. That is why I am trying to help him. The problem is not the time but the "BASS". I sleep late and the volume at which I play during nights is real low. But the issue is there are some parts in a video which have sharp bass and other parts dont. So no matter how low the volume is, it irritates him. So I guess it would be better to go to his house and do some tests to find out his tolerance limit.
@Fire-Fist:
change your apartment or get rid of your speakers
Yeah thats a solution you got there!
