The crossover point is not the frequency that the driver suddenly stops making sound, but the point where it is 3 dB down. It takes at least 10 dB for one sound to drown out another, so the useful range of that driver at 6 dB per octave is at least 500 to 4000 Hz. It is an unusually small range, but maybe the mid woofer and lower tweeter just didn't have enough musical overlap to work, so this dome is just bridging the gap. This is a range where many systems sound harsh, maybe because the drivers are working at the limits. By putting a dome here, maybe it allows the other drivers to work more comfortably. Are they Thiels?
1st order crossover?
The speakers I have, have a 2 inch dome driver that has crover points at 1,000 and 2,000. In the owners material and set up a lot of attention is given to phase and time. Each driver is in it's own cabinent and a measuring device is used to align each cabinent after measuring the seating position from top of sub and floor to ear distance. Can the crossover points for that 2 inch dome be 1st order "6db per octave" when it is not even covering an octave. It has been said that drivers for 1st order crossovers have to cover such a wide frequency range. That is not the case here. Hope to learn a few more things on 1st order crossovers. Thanks