As is mentioned on the Leslie description page, a common modification is to remove the conical diffuser from both active and dummy horns of the treble rotor. This will cause an increase in the AM component and a decrease in the FM component of the sound. As you can well imagine, there are opinions on both sides of the value of this modification and you might want to listen to one before you hack away. On second thought, a new horn assembly is not too expensive, so you could buy a second stock horn and hack away at the one you own now. That way you'll own one of each.
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