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The best sounding tweeter I have ever owned was in a speaker made by Harbeth, it used a Seas aluminum dome with ferrofluid cooling. Cymbals had a magical shimmer to them, midrange had additional detail and the integration between woofer and tweeter was seamless and coherent.
Aluminum domes often take more work and crossover parts to make them sound so exceptional- namely the need for low pass filters on the very top end to squelch any rising breakup distortion.
Get them right and they are hard to beat. Get them wrong and they are impossible to listen to.