Pushing the envelope here, but...
In case this helps others: The Mytek (and I suspect most/all DACs) can only receive a single sample rate. If using one DAC for two zones and playing different sample audio in each, you need to convert the sample rate of for one or both zones so they are the same:
DSP/Output Format. Note: to set all to a single sample rate, right click in the box. If you set both zones to convert all to the same rate, everything will play without conflict.
MATT: The above solves the issue described. However, for critical listening times for the best sound quality, am I better not doing sample rate up-converting?
Thanks again