Been busy at work, and been meaning to chime in for a couple of days, but no time until now.
Really enjoyed what you wrote, and the response you quoted over at the prosound web, and for me, there's a couple of great points made by Michael, which need to be put in context in regards to Bill's designs.
So, please stick with this a bit, but:
1. Bill's designs, and the consequent SPL (ie response) charts are raw data charts.
That means there is no "colouring" of cab response due to the active module attached to the cab, +/- EQ colouring (eg FOH or monitor with a switch). The cabs that have this are claiming flat response out of the box, and, largely, that's true for them.
2. Because the charts are raw data, and the response levels in dB may be different scales between subs and tops, there is no tilt toward the bottom end. In fact, if you want to work out a entire response graph including subs and tops, you need to extrapolate response from each chart and also factor in reductions based on slope type and severity.
3. This is in no way a negative comment. It is meaningless to include a target response curve unless you include the native response of the cabs the target is for! You can't understand one without the other. Michael categorically stated what his target curve will be, but not the response of the cab(s) in the first place, so you can't understand what he has done (other than if you reverse engineer response based on what he said he tries to do).
4. Does mixer response vary from brand to brand? Sure does.
I applaud you for your interest and research. I do believe though that certain things are absolutes in PA's, and EQing.
You have to know the response of your cabs without room colouring, ie compare expected SPL chart response, to your own measurements outdoors.
You have to end up with a system response that you know and consequently use as a target curve for your system indoors. Invariably, indoors response is dictated by the room. So if you know the outdoors response, you are only changing it based on the room.
Your target response (outdoors) will be what it is, flat. As Michael suggested. Or something else someone suggested.
For me, target response is the native cabs response outdoors, flattened by mirror imaging +/- tweaking by ear. I'm getting better all the time with this. You will too...