When a driver is used on a horn, its response is changed dramatically. Much of what you see as rising response in a direct radiator is caused by collapsing directivity, as it goes from being an omnidirectional source to being more and more directional as frequency goes up. A horn sets the directivity and changes that.If a direct radiator has a flat response curve on axis over a very wide frequency range, then it actually reduces acoustic energy as frequency rises. This compensates for collapsing DI as a direct radiator, but it also reduces output at high frequency when used on a horn.