Isolating your sub from the floor won't help much. The long waves of the bass are going right through the ceilings/floors above you.
Work with your neighbors. In your room, watch an action movie at your preferred volume. Without changing the volume knob, turn off your receiver for a moment and unplug your mains, center and surrounds. Now when you turn your receiver on, you'll only hear the sub. Call your neighbor on the third (and preferably the second floor) and use the subs' rear-panel volume knob to keep turning down the sub until it's not bothersome to them anymore.
Now turn off your receiver again and hook up your other speakers again. This is the output that will be acceptable to them.
If it's not acceptable to you, you have a few options:
Tactile devices like Buttkickers, headphones, watching action movies while the neighbors won't be home or inviting them to watch the movie with you (especially if you're each single, etc....)
