The apparent increased highpass steepness at 20kHz crossover could be some combination of tweeter passband inaccuracy, room response, reflections, and, the thing I was obsessing over recently, the supertweeter's own crossover. It makes sense that to make the Behringer highpass interact as little as possible with the Elac's built-in crossover by moving the Behringer as high as possible. However, what one wants is correct operation, not necessarily minimal interaction. But somehow the minimal interaction seems to work out give desired or at least desirable results, which can simply be compensating by raising the level. (Or lowering the level of the other drivers.)
Oh, yes, and it could also be measurement error. I haven't been showing the impulse response, because it looks terrible. My purist friends should be shouting "told you so". The Acoustats by themselves have a very nice impulse. The supertweeter, right now, is simply messing that up badly. Which then brought to my rationalizing mind the question, what does the perfect Tact response look like?