I'm a little confused simply because I may have misunderstood what to do with my riser. For the riser, I had thought that the drywall should go all the way to the concrete floor (with a quarter inch gape that will be acoustically caulked to the concrete), with the riser added later to the interior of the room after the double drywall is up. I was thinking of my contraption in the front of the theater as a mini-riser of sorts, which is why I was assuming it should be installed the same way. Is my plan for the big riser correct? What is the advantage of attaching the drywall to the baby riser?That is the norm. However, regular 3/4" sub-floors are nailed down across joists, such that there's no gap between the joists and the sub-floor. So, in your case the potential problem is you're elevating the new sub-floor every n". Does that make sense? The pressure of whatever is sitting on the sub-floor + the sub-floor material itself are applying gravitational force at only the points where your 1x4's contact the underside of the sub-floor.
So, first if you strengthen the thickness of the sub-floor, then the sub-floor itself becomes more rigid and is better able to handle what are called point loads without flexing. Secondly, you will have a consistent load on the sub-floor above its outer edges, created by the bottom plate of your inner stud wall. By utilizing 1x4's beneath the edge of the sub-flooring, you would reinforce the ability of the sub-floor to transmit that vertical load down to the concrete foundation.
I hope that's helpful and not confusing.