It's hard to believe that RF may be the cause of this but still possible. Even though the horns are plastic it is possible that they have some shielding around the amp. It also possible that there is a cap or resistor difference in the amplifiers that could make one react to a RF frequency while the other works fine. If a ceiling speaker and horn have both the audio and power connected in parallel and it only effects the speaker then that eliminates many components. Have you tried to shield the ceiling speaker amp?

Merritt

Business Telephones & Equipment + Commercial Audio/Video Products
Commercial Communications . . . Turner, Maine
If it was built after 1980 don't expect it to work right.