Acoustic Treatment Suggestions?

I've been building a moderately sized project studio in a house with some other tenants living in it so sound reduction is a top priority. Our drum room measures 8' wide by 12' long and 6'3" high. (I know these are not good dimensions, but try to look past that) It consists of a wood frame with homasote, fiberglass insulation and volara foam layered on top of each other. Inside there is decoupled hardwood floor with foam and hardwood diffusors on the walls.

Between the acoustic treatment and the walls of the house, a drummer is clearly audible but not obnoxiously loud one floor up. Its just quiet enough to have a conversation at a reasonable level. Most of the highs are attenuated well, but as I was expecting, the lows are not.

I've been told by a few people that layering different, thin layers of mass loaded vinyl and other high STC materials is the key and others have said that to have any substantial effect I'd have to make my walls 6 inches to a foot thick, which would take up a lot of precious space. What can I do to substantially reduce the transmission of sound (particularly lows) to the outside of the room?