Not sure what your question is, but the REW targets are a standard 6dB down at the crossover frequency. As you know, if you combine two signals at the same level and frequency you'll get a 6dB gain as a resultant.
So in a perfect world, the red line (shown below in the pic) would be the resultant of a set of mains and sub when combined and given the standard crossovers. A perfect flat response...
80crossover sub plus main.jpg
brucek