Hi Nico, my guess is they're the victims of a loud party...:).
Anyway, i assume you're able to do someting with electronics i can advice the following: just take them out of the speakerbox (only 2 screws per tweeter) and measure them with an ohm meter / multimeter.
If the reading is like 4-8 ohm it's probably fine, if it reads infinite the tweeter is dead.
If you don't have any multimeter, connect them to, for example, a small radio on low volume (!!) and see if they make sound still (of course: if they still make sound they're probably ok).
The avant has separate amps for low and high per channel, it's unlikely that on boths channels the hi amp died, so my guess is the tweeters.
If you find the tweeters are dead, i have spares for you (and i live in NL too).
Good luck.