Thought this might be interesting for you guys - the Chinese forums tested the THD of three IEMS- UM Mirage, Mentor, and Beyerdynamic Xelento. Turns out the dynamic Xelento has way less distortion than the BA IEMs. The fully open mid-range driver on the Mirage has higher distortion than the stacked mids drivers on the Mentor. The 4x high driver on the Mentor seems to give it really low distortion compared to the 1 driver Mirage.
UM Mentor V3: 12BA, 4-way, 4 lows, 2 low-mids, 2 mid-highs, 4 highs, 5-bore (ends with a metal tube) with 1 bore for dB-Go module.
UM Mirage: 3D printed titanium shell, 3BA, 3-way, 1 low, 1 fully open mid, 1 high.