I wonder that so far there was no mentioning of, and this is especially intended for you gentlemen from USA, the circular ribbon tweeters from San Diego. Of course, I mean Acoustic Zen circular ribbon.
My HI-FI vocabulary is very poor, so I give you something from the man himself, Mr. Robert Lee:
My circular ribbon tweeter design, unlike a traditional dome or cone tweeter, presents a purely resistive impedance that's completely linear in the audio range. It also has an essentially linear phase response, which contributes to an immediate and precise response to any transients in a complicated music signal. This dramatically reduces distortion and coloration.
I am using the thinnest diaphragm possible, a 0.01 millimeter Kapton diaphragm. That's just about weightless, with a 95% covered aluminum circle of conductors across the entire vibrating area positioned between super strong Neodymium magnets. Some people use metal ribbons which can result in very harsh HF response. Kapton is not only extremely light, it is also heat resistant, hence the thermal behavior of the voice coil will not translate into audible distortions. My tweeter's larger circular membrane not only provides much higher power handling and a wider frequency response, it also eliminates the offsets between horizontal and vertical dispersion that are common with narrow ribbons.