sh: You're completely right with the 'more high frequencies' with horns because of the directivity feature. (Apologies - had let that slip by). So the showing up of hiss could indeed be signicantly more with directional speakers.

As for your other 'horn' comments: 'exaggerated' horns can be pretty 'bad' - and most should be relegated to PA - a quality tweeter with a 'small' exponential horn built as an extension of the tweeter's own dome can produce very good - and high fidelity - sound.