A couple of weeks ago I put some exterior door weather seal around the door and a door sweep to my basement where my HT is located. I think it cost @ $20.00 from HD.
It accomplished three things from what I can tell.
1. It reduces the amount of sound that goes up into the rest of the house through the stairwell and door.
2. The door does not vibrate from the sub energizing the air.
3. It helps seal the room for the sub to energize just the basement. When you close the door you can hear a difference, albeit slight.
I still have to finish paint the wood door stop but here are some pictures.