Dave,
I'm not saying that I disagree with your sub floor. If you built it the way you discribed, you did it right. It must be solid. I'm just not sure if it really has an advantage or not.
There is nothing special about my block walls, except they are built like your sub floor. Cinder blocks, 2x4's on sides, and foam panels in between, but covered with drywall. I can go outside the room with the all 10 speakers plus the subwoofer cranked up and place my hand on the outside of the walls and feel almost nothing. I have another room in my house that has stud built walls where I have a two channel horn system without a subwoofer. When I go outside the house and put my hands on the walls I can feel them flexing. I can also hear the music almost as well as I can when I'm in the room. I can't turn the volume up as much as I can in the theater room, because of room distoration. The walls and doors are moving the windows rattle, it's not good.
The theater room is 17' by 30' and has grown too small for all the speakers. I'm now drawing up a new room that will be 24' by 32'. I'm not saying the size is perfit for my system, but it should be better, and is all I have room for. Now you have me wondering if I should try your sub floor.