Hello Ravi,
I might as well be plain-spoken about this to save you a lot of grief and effort.
Short of building a room within a room, at enormous expense, there's no way to retro-fit a room and make it "soundproof". The methods used by pros require gutting and rebuilding, or starting with a new building (like a house). Staggered studs, double layers of wallboard, dissimilar materials, double floors, and heavy, sealed doors with weatherstripping--all of these go into making a room truly soundproof.
That said, there are a couple of things you can do to lessen the annoyance to tenants living below your apartment. You can place your subwoofer in a shallow tray with alternating layers of gravel, sand, and scrap carpet. Any "sandwich" of dissimilar materials will work--bubble wrap, carpet, and plastic foam. This will prevent direct mechanical transmission of low bass through the floor to your neighbor's suite. (Keep the cat away from the tray of gravel or it might do you know what!) The same approach will work with large floorstanding speakers. But it will do nothing to suppress acoustical transmission of subwoofer frequencies, so go easy on the level control. Obviously, a heavy carpet with underpad on the floor will help.
Anyplace air can get through, sound will follow, so weatherstripping on the apartment door will prevent some sound leakage into the hallway.
Regards,
