Here's how I did it:
I made a small low-frequency microphone amplifier, hot glued an electret microphone to the floor, and made sure there were no leaks around the microphone (to avoid audio-feedback). I connected this to an active subwoofer (this is a subwoofer that has an power amplifier built in, typically the thing you have in your average surround amplifier, and placed the subwoofer in the room next door to the mike (to avoid feedback!). Now — it's very important to disconnect any surround speakers as you ONLY want the BASS to distribute into the structure of the building itself, this makes it IMPOSSIBLE to locate the sound from YOUR flat, and the penetrating middle-tones that comes from your noisy neighbors stereo — will reveal that it is YOUR NEIGHBOR that plays the loud music.
It worked flawlessly, the entire building shook with the sound of the neighbors stereo, and the teenager had to turn down the sound several times due to multiple complaints from the ENTIRE neighborhood.
Revenge is SOO sweet