The specifications for many of your speakers go out beyond 20kHz? What is the advantage of this?

Generally the frequency range specified indicated the limits of where the performance has started to diminish. Many dome tweeters that are specified to extend out to 20kHz are already up to 3dB down at that point. By engineering the tweeter to extend to over 25kHz, we make sure that it has not already rolled off by 20kHz.