Rated at "109 dB in their passband" I'd have thought 40 WPC would be plenty enough to make them happy. But this weekend I moved them up to 100 WPC and the harshenss is gone - you can crank them to 11 without adding any strain whatsoever to the sound.

Another point is that the Altec horns were made to be screwed down, but most people just plop them on top. With the flange screwed tight to the bass box there's a world of difference, even without additional damping of the horn itself.