I just acquired four Fountek FE87 full-range speaker drivers. I am always on the lookout for midrange drivers that would be suitable for a midrange horn. I did some free-air listening to pure sine waves from 27Hz to 880Hz and can detect no buzzing. This is a plus, because if your horn driver has any buzzing, the horn will amplify the buzzing right along with everything else. Here is an image of the test setup:
Fountek FE87 with Plaster Midrange Horn
I have tried other small full-range drivers but even the new ones seem to buzz throughout the frequency range of interest. The Fountek FE87s I have tested so far have a very clean sound.