The only way to soundproof an apartment is sell all your gear. Seriously. The bass will be louder in the apartment below you than in yours. Your subwoofer is coupled to the floor so the ceiling in the apartment below you will be a big sound radiator. You could try some isolation feet for the sub that might help a little bit, but to do any good you'll have to spend some major bucks for them.

Problem is, the low frequencies will still carry and everyone around you will hear it and it's very annoying because all you hear is the BOOOOOOM BOOOOOOOOOOOOM just like one of those car audio systems that the kids have.