I use Dante instead of USB….but the setup *should* be the same.
Save the Eventide’s USB 1+2 inputs as the ‘main outputs’ of Ableton.
The other 14 inputs / outputs will be your sends. Set up a direct I/O between USB inputs 1+2 and whichever output on the Eventide you want to monitor from. An example USB 1+2 connected to Analog 1+2
This way, anything coming from your ‘main mix’ in Ableton will get routed through USB 1+2 to the Analog outputs directly.
I do it this way only using Dante…where my main outputs of Logic (output 1+2) correspond to Eventide Dante input channels 1 +2. The Dante input channels 1+2 are then routed directly to the SPDIF outputs. From the SPDIF outputs I have headphone / monitor controller setup.
Hope this helps.