ribbon tweeters

This is more complicated than you may think. The crossover needs modification and someone who really knows what they're doing needs to do it. A theoretical crossover point and curve may or may not work. I'd not do it. Get different speakers. I understand why you'd want one as my ribbon tweeters are as good as I've ever heard.

Well, the sensitivity of the ribbon would need to be a close match to that of the other drivers, or a little higher so that the crossover could be adjusted for proper balance. Also, the crossover point would need to be considered or reconsidered...

Short answer: One size does not fit all. If you just blindly pop any ribbon tweeter into any given speaker system it's going to sound like that's exactly what you did. :)

You have to redesign yr spkrs. Forget it!

Pick your ribbon and write to Bill Legall an absolute MASTER repair replacment man. If anyone an do it he can and adjust your crossover if its possible. Millerspkr@aol.com. I am not affiliated with him except I consider him a friend.
He really is known nationally and will tell you what can be done. Be prepared to pay some money he's relatively inexpensive but what you want sounds expensive,

Compare any ribbon to the Seas T25.
Ribbons I find to be so fake sounding.

It depends on what the speakers are. Are they expensive prised posessions, or something you would like to play with? If the former, forget it.

The idea that changing out a tweeter requires an advanced engineering degree and an exotic European name is absurd!

Tweeters are not critically matched to the enclosure they are mounted in, (as woofers are) and therefore are much more suitable for experimental changes. The replacement tweeter should have the same impedance as the old one, and have a low frequency range as low or lower than the old one. Having the same sensitivity would be nice, but this is easily adjusted by changing the tweeter padding resistor of the crossover. Read up a bit on crossover networks because you may want to diddle it a bit.

It ain't rocket science! We have very sophisticated mathematical methods and computer tools that let us design speakers so that, before they are built, we know they will sound good. Then, after they are built they are tweeked, to a considerable extent by ear, so that they sound excellent. Since tweeking is usual in any case one can start with that, especially if you are working with an existing speaker that is necessarily "in the ball park".

Fear not! Have fun! Become an expert!

The speakers in question are Mission 770, that I picked up for a song. The tweeters are not originals(anyone know what they were?)so I was thinking about upgrading them. I have heard a lot about the speed of a good ribbon,so I thought why not give one a try. I was thinking about the Hi Vi ribbons from China.Any other suggestions for replacement tweeters is most welcome, and thanks for all the help so far.

Ribbons cross over at a higher frequency due to distortion limitations. This is one reason that you see many ribbon "supertweeters". Domes typically measure better mainly because of the distortion at lower frequencies. So your crossover would need to be changed, a resistor added or value changed will not cut it. Also the Hi Vi are not very good ribbons compared to the BG neo3 or the Fountek, Aurum Cantus etc.

The speakers in question are Mission 770, that I picked up for a song

Jeeez, I thought you were referring to Martin Logan (mentioned in yr "system")!!
Of COURSE you can play around. Hi Vi & B&G could be (cheap) contenders -- for a more expensive (real) ribbon you can check out offerings at Madisound & Zalytron.
You must find the xover's hi-pass frequency & filter slope & the present tweets' impedance (at the xover frequency -- or the nominal one).
The most difficult part will probably be fitting the new tweet to the opening...

The best thing is to run ribbons together with a dome tweeter.This gives the best of both.I note Dali is now doing this.The ribbons sound best above 7k.Try using just a 2.2 mF cap for an 8 ohm ribbon and sit it on top of your speaker.

JT