The midhorn's range allows the use of the standard 1.6kHz tweeter crossover. With the acoustic rolloff of the midhorn, you get perfectly matched curves when the midhorn and tweeter mouths are flush. Some drivers need a small coil, some don't. Each driver is independently optimized for perfect summing taking position and electrical characteristics into consideration. It really sums nicely with seamless transition between midhorn and tweeter.

The crossover was designed to use the Eminence H290 or π wood tweeter horn. Other tweeter horns may or may not sum properly, especially if length or dispersion characteristics are much different. For example, the Altec 811 will probably need to be positioned differently to get summing right. You'll probably need to move it forward a few inches.