1st, know the Horn's Angle of Dispersion
Some horns are designed for wide horizontal dispersion, say big room, stadiums, and many horn speakers orient the horns that way, not best for home listening which wants narrow dispertion for tight imaging, minimizing early side wall reflections. It's not just horizontal or vertical orientation, it also involves the horn's specific mouth design.
Others are specifically designed to produce narrow dispersion, like this Electro-voice T350, designed for their vintage home speaker systems
https://products.electrovoice.com/binary/T350%20EDS.pdf
Vertical and Horizontal Polar dispersion graphs showing volume fall off in db used to be standard for vintage drivers
https://www.ravepubs.com/a-deeper-dive-into-loudspeaker-directivity/
tight width and tight height dispersion also minimizes early floor and ceiling reflections.
tilting vintage home horn drivers is important, aiming tweeters at seated ear level, and toe-in so the horns face the listener, to work properly with the designed polar output.
The big Klipsch horns, the horn is horizontal, and is designed to disperse the horn's dispersion at 45 degrees into the space, away from the side walls, tight height output, into a LARGE space.
Many horn designs today orient the horns for too wide distribution, weakening imaging, producing early and too strong side wall reflections.
So, ascertain the horn's dispersion before moving on to other factors, listen for tight imaging, lack of early reflections.
