What's the best-sounding tweeter that can replace...

...typical dome/ring-radiator tweeters with a round mounting plate of around 4"?

I have Audio Physic Avanti IIIs and a VonSchweikert Visiun centerchannel; each has a ring-radiator tweeter in it. The quality of the treble is at least OK, and it'll get better when I replace the series caps in front of the tweeters, but I fantasize about replacing the tweeters with something...airier...for lack of a better description.

I've heard excellent-sounding systems with Aurum Cantus and Fountek ribbons in them, so maybe I should consider those.

Comments? What you you LOVE?
Do you have measurement equipment. Becuase unless you do I wouldn't touch the system. Those are expensive speakers and without measurement equipment even if you fit a "better" tweeter you are unlikely to get a better result.

However if you do, I would look at something from the Scan Speak revelator series. Eg.

R2904/700005 - Scan-Speak 1 inch soft dome tweeter - Europe Audio

This uses a similar construction to your existing tweeter but it is unlikely that even at the cost of your speakers they could afford to fit a tweeter of this kind of quality. Weather it is actually any better is another matter but at least the designers had more money to throw at problems.

As for replacing with ribbons. You may need to get into a significant crossover redesign to make these work as they will most likely have very different characteristics to your existing tweeter. I wouldn't try this unless you have experience at reworking crossovers, a measurement system and a speaker simulator or lots of time to learn and are prepared to buy the above. (You can get most of it free apart from the microphone)