I mean broadband line source array.
Because from what i know, LAcoustic was the first implemented broadband line source array.Though it's not really broadband.
It also depends on how you want to determine that.
It was very common practice to do what was called "stack and splay" Where the horns would be stacked and slightly turned for coverage. Sometimes the mids.
But if you are talking about simply stacking full range cabinets on top of each other-that is a different thing.
Where the current "line array" theory comes into play is where Christian Heil was actually addressing the interactions between elements and boxes-spacing etc.
That is something that very few earlier had considered-in a vertical setup.
So as usual-it depends on exactly what you mean.