gnoworyta, no matter how this sounds I am just trying to be helpful. I realize you are trying to find the cause of your problem, not repair of the tweeter. I was just looking online and it seems the most common problem with ribbon tweeters is that the tweeter material, which appears made of an aluminum foil type material, rips. From what I read, if your problem is a ripped ribbon, it appears to be an easy fix for a professional, assuming it is not an exotic material. Is it possible to visually inspect if it is ripped? Also, if you can remove it from the speaker maybe you can isolate if it works ruling out a crossover issue. I hope some of the guru's here can help out on how to test a ribbon speaker detached from the speaker cabinet.
Here is a YT of a RAAL being repaired.
https://www.youtube.com/watch?v=bvylZF9eJOAIt may be your dealer knows that something else is burned out within your tweeter making it totally unrepairable. Like maybe a ripped ribbon tweeter still makes some noise, or whatever, and your doesn’t. But if your saying its $800 bucks for a new pair I would definitely investigate further. Millersound is first rate, give them a call. When my woofers needed repair he basically said he can repair most anything on the woofer regardless of problem.