Hi Joe, nice Sketchuping. In general principle, building a wall to create a wall/space/wall system is very effective. A two leaf plasterboard and staggered studs, with each layer TOTALLY separated from the other, will perform as well as concrete, but the details need to be impeccable. One hole leak or contact link from skin to skin and it's all wasted. Brick walls are usually compromised because the block layers do not entirely fill each gap. Lots of bubbles in the cement is not the same as solid concrete. Either way the most important factor is that it be done right. Less than perfect will certainly fail. You need to get the people doing the construction talking to us to be sure there is understanding. Or you provide the link.
Don't remember where I got this, so anonymous thanks. Says it all.
To be sure of success, I suggest that you carefully investigate the travel paths of the sound. It can be very difficult to identify what is coming from where, particularly with Bass. That corridor is already a Wall/Space/Wall system. Why is it not stopping sound?
So before you build, look for leaks. Put draftproof seals on doors. What is happening overhead? Sound can easily travel up into the attic and down again. There are lots of possible what we call Flanking paths. Sound is ingenious at finding a way.....
DD