I think what they were really pointing out is that generally ribbon tweeters do not have as good as off-axis performance compared to domes. However, there are ribbon tweeters out there that trump down tweeters but generally you can get significant performance from a dome tweeter at very little cost which is why manufacturers tend to use them.

Ribbon tweeters have a tendency to be more expensive which is another reason why you see them only in so called "high-end" speakers.

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