But your design didn't use the ED29689. WBFK always create a dip in the frequency response when used together with that driver as Peter said.
No, not always, but if you want to preserve peaks for ear resonances you'll probably end up with deep around 6kHz. I'm afraid moving drivers away from each other won't fix it unless the distance is really big (by big I mean >2cm)
if you measure with iec711 than valleys after 10kHz is quite typical for most drivers - but not every.