I have the exact same problem. It’s significant on the left speaker and subtle on the right speaker. But the distortion is definitely coming from the mid range driver. I did disassemble the mid range driver that is severely distorted and found no deterioration at all in the Dome. I reassembled and it did not improve.
I have also attempted on multiple occasions to contact Avalon both by phone and through their help email system. It’s been weeks and I’ve had no response whatsoever, other than an automated response immediately after sending the first email.
I have carefully eliminated other potential problem areas such as preamplifier, amplifier, and crossover networks. All my gear is Mark Levinson. Distortion persisted independent of audio source.