T20 has now been measured:
Red and blue lines represent the output of each driver individually, black is combined. You can see that the horn is much less effective with just one driver and the difference is far more than expected. Running together they smooth each other out. The difference suggests that the passive driver when switched off, actually undermines the horn. The plot above shows the response unfiltered.
Blue: Lower crossover
Compared to simulations:
Simulation shown in red. You can see the peak is higher in reality and has more damping. The real version rolls off earlier on the bottom end. With room gain however, the response is flat to well below 20 Hz, even with high pass filters in place. In short, this sub is now a beast!