If you can't get the original replacement drivers I think it's time to go shopping for new speakers. But maybe there is something wrong with both drivers that is repairable. Why not remove them and investigate?
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Did you try different components? I had the same problem with my Thiel 3.6 speakers distorting certain solo piano notes and Thiel said they could rebuild the driver if I remove it and send it to them. However, I picked up an SACD player and the distortion did not appear when playing the same disc. Turns out it was a bad polypropylene cap in the analog board of my Theta Gen Va.
Normally, the dome in the center of the driver that protects the voice coil from dust and such. I just looked at pic of your speakers and see that it has a large single dome like the mids in my Thiel CS5i's. In that case, try different locations on the dome. Be VERY careful, let the adhesive properties of the Dynamat backing do the work of holding it in place, do not press lest you dent the dome. Just lightly touch it to the dome material.
So the first obvious question is, have you reached out to Avalon? They may have replacements for you in stock.
Next, I don't hear about midranges going out very often. Double check to make sure it's not a crossover thing. Examine the crossover for two possible items:
1 - Burnt resistors
2 - Electrolytic caps (anywhere).