I would say a well designed metal tweeter(titanium, aluminum, berylium ect) will play with more detail(crisp) to higher freq's than fabric type tweeters which are often known to roll off the highs somewhat. To some people this may mean "smoother", to others it means the sound is "coloured". The key with metal tweeters is that they are designed well, a poorly designed metallic tweeter may sound rather harsh at higher fq's.

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