German loudspeaker DIY magazine Klang und Ton
http://www.klangundton.de developed a fully passive three way OB with Alpha 15 A and Beta 8 CX with EAPT 50 coaxial horn:

OB is 58 cm wide and 90 cm high. Frequency response is rugged, but on level. They say that the 600-700 Hz notch disappears off axis:

XO frequencies are 350 Hz and 4 kHz. You will understand that I cannot publish more details (like Xover schematics), but I try to be helpful if somebody shows real interest.

Rudolf