Sound Proof???

The closer you move to the speakers, the less volume that you will need to enjoy the sound. This will also satisfy your neighbors. Sitting five feet from your speakers is a bit unconventional, but it's a good compromise.

6month:
How loud do you listen?
If bass is your problem maybe for the time being you should do some reverse engineering using some accoustics, try to evaluate if your listening position is in a node or a null if you're seating in a place where room produces less audible bass you will naturally try to compensate increasing volume or your sub setting. You might need to compromise between bass amount/quality and lessening problems with neighbors. Find where bass response is hihger than current position and try to position your chair or move speakers sub in order to get the best possible response at your listening position with less volume so you might decrease neighbors annoyance. What do you think?
Luis

Another great thing you can do is move your speakers and/or sub(s) off the floor and either "fly" them using hooks and vinyl coated steel cable or Omni-mounts with rubber/vinyl pads between the speaker and mount and the mount and the wall. This is how they mount speakers in Pro A/V applications. Your neighbors are complaining abour mechanical vibration in the low Hz, so if you keep your speakers up the airborne sound transfer into mechanical vibrations will be at a reduced amount.