If your horn has 80Hz flare rates (or least what Bruce states) then 80Hz order high-pass it too high. I would start form ~40Hz to unload the LF form it. However, I would be questioning if it really were necessary. First of all this horn has back chamber that changes the rules of the game and marks the horn less sensitive to the overload the flare with LF. Second, Bruce used the fairly low Fs driver in there with large trout that makes them to act at lowest cut off as the direct radiators…. I very much doubt the it would be necessary.