I have seen other horns (Jericho 08) that use a small internal helmholtz resonance chamber to flatten the horn response. Outside the first flare section between the dog house and the back wall there are two unused closed spaces. Have anyone tried to use them as IHR and reduce the LS peak at about 150 Hz?
In a PA setting it would make more sense to EQ it and keep the radiation resistance but in a domestic setting, why not trying to tame the peak?