I am buying a new pair of speakers. Should I buy speakers with a soft-dome tweeter, or ones where the tweeter is made out of a hard dome such as aluminum?
Some generalizations about tweeters can be made. For example, metal-dome tweeters such as aluminum tend to have higher-frequency break-up modes than, say, silk-dome tweeters that will start deforming at a lower frequency. However, when the aluminum dome breaks up, it does so much more severely and this usually shows up as a spike in a frequency-response chart, often within the audible range. Silk-dome tweeters break up more gently, and some feel that makes them more pleasing to the ear even when it's still in the audio band. Another generalization that's made today is that the best tweeters use beryllium and diamond for their domes. Their break-up modes are super-high.
The truth of the matter is that while there is some truth to that stuff, a lot comes down to the implementation of the technology and, overall, how the speaker has been designed. Editor-in-chief Jeff Fritz recently told us that he compared two speakers, one with a beryllium-dome tweeter and another with a dome made from silk. They're both outstanding speakers, he said, with high-frequency performance that's roughly the same. Moral of the story: put most of the weight on what you hear, not what the dome is made from.