I would be curious as to what is causing the 12kHz peak...if it's a resonance or if it's just the natural rising response of the driver which isn't a bad thing it simply depends on how your using it.
The 10F/8424 in a front horn is excellent. %THD is well under 1% at 250Hz and drops to only 0.10% from 500Hz-3kHz. Subjectively it has a very balanced sound that is both lively and smooth...with no breakup until around 18kHz. In a non horn configuration (direct radiating) the best the 10F/8424 can do is 0.8% through the midrange. %THD is well above 1% at anything below 500Hz with direct radiating.
Zaph and Hifi Compass both measured no more than about .5% thd above 500hz. All depends on the reference volume level relative to mic distance