If it is a back loaded horn, you can adjust the compression chamber throat to balance the frequency response. The stronger the magnet, the smaller that the compression chamber and the narrower that the throat needs to be to achieve a balanced sound. I have my Lowther DX4s mountd on open baffle. This is the most transparent sound for them, but then equalization is required. With mounting them on open baffle, even with the the passive components in the signal path, the sound of Lowthers is more transparent than mounting them in back horns without such devices in the signal path.
Retsel