Having an ultrasonic peak is not unique to the MA tweeter - most metal-dome tweeters have such a measureable ultrasonic resonance point, of differing magnitudes and frequencies depending on the design. It is assumed that the higher this peak can be pushed up in frequency and the lower it can be pushed down in magnitude, the better. It is also assumed that any deleterious effect this peak may have on the sound would be more caused by possible interference artifacts, reflected lower down the spectrum, than by the peak frequency itself (which is, let us remember, above the range of normal hearing). The perceived 'brightness' range actually occurs below the top octave (below 10KHz), well away from a 22KHz peak even for those few listeners who do hear up that high. With typical CD sources, the steep HF filtration usually employed by that format would tend to make tweeter resonances at or above 22KHz a less suspect issue.
