You mentioned that a horn design is not used for mid/HF cabinet design, is that because lower frequencies will bounce more effectively than higher frequencies?
I ask because I want to design a practice guitar amp (for my A level DT project) which incorporates a horn design in the cab. It would (in theory) mean the sound is quite directed, i.e. you sit in front of the cab and you can here perfectly, loudly; but if you sit diagonally etc to the cab then you can't hear it nearly as loud... this would mean easy practicing at home - happy neighbours!

As far as I know, guitar amps use only 1 driver to handle all frequencies... they aren't as bassy as a bass guitar amp obviously but nevertheless still have a fair kick. I have a donor practice amp, so basically all I am designing is the horn cabinet for it.
What do you reckon the outcome will be from using a horn in a guitar amp?