Are my " diamond " tweeters really diamond

So true. Not all soft dome tweeters are good. Speakers have to be voiced. Makes me wonder who they are voicing these things for. Certainly not me.

I am fond of electrostatic tweeters, myself.

Breakup is important. You don’t want to drive a transducer close to or beyond breakup. This is something the designer must take into account.

The key to soft domes is that they are intrinsically damped - doped fabric! This is the key! It means that the dome adds minimal coloration to the music. This is why they are so revealing - you hear the detailed music after a transient rather than the cone vibrations after a transient. Some folks are more critical listeners than others, so not everyone will appreciate this. You can see this on a waterfall plot, the excel millenium fabric dome tweeter has the cleanest waterfall plot I have ever seen. Some metal or rigid domes come close but the extra frequency extension of rigid domes is totally at the expense of clarity where it is more important - at upper mid range frequencies.

A breakup node that is outside the audible band will impact the audible band by IM distortion. Using red book, most metal domes will have a breakup above the FR range. Analog and true hires recordings can have energy that may cause the driver distort. Typically, these ranges are lower in level and thus the IM distortion will be accordingly lower. The use of exotic materials is one way to push the breakup out the range of most content. Another is engineering design, which can take a material quite a far with shape, variable thickness and stiffness, and other design features.

Soft domes will breakup quite a bit earlier, and the exact degree will vary based on the doping materials. The distortion will be more benign due to the internal damping properties. Other distortions are more present with soft domes as compared to other more rigid domes, but the internal damping does help to keep the issues generally benign. You don't have this kind of forgiveness in most rigid domes.

The exotic material choices is an effort to get a more ideal driver. Still, its only a single piece, or a single chapter, in describing a drivers performance.