I am planning to put up a layer of QuietRock on top of existing drywall, potentially with Green Glue between the layers for good measure. I am doing this because the wall in question is against my neighbor's bedroom wall and these condo walls are thin. Since I am going to be relocating my computer's subwoofer to my room after the job, I am probably going to need some sort of sound absorption for the subfloor. Since my room is on the second floor, bass is most likely going to travel along the floor and under the QuietRock, thus defeating the purpose. If it were on a concrete slab, I probably wouldn't have any worries.
Would mass-loaded vinyl do the trick in dampening floor vibrations from a subwoofer?
Edit: I suggest the vinyl because I am not as concerned about bass waves traveling through the floor since both of our garages are directly beneath us. It would seem that the vinyl would work in keeping waves traveling horizontally under the drywall. Knowing what to do here will determine the order in which I proceed.