Here are a couple more pictures with 3 and 4 boxes stacked. The bottom of the bracket is definitely going to need to be extended another 6" or so to help center the weight as Matt suggested earlier in this thread.
Another issue is going to be the stability of the PA stack, by the time you get 3 tops on 3 subs, you are getting pretty tall with a rather small footprint.
I believe I have enough metal to build one more prototype with the longer base, now I just need to find a bit of time to get another built.
3 Boxes, Angle of the top of the top box is 10*
4 Boxes, The only situation I could see this being useful would be a very small outdoor stadium section covering bleachers. We used to do something like this for graduations, but with bigger boxes.