What are xruns?
it's a buffer under-or-overrun, X stands for under or over. It's a sign that your system did not process some buffers in time, so some data is missed. It is particularly true when you run at very low buffer size where the sound card should process incoming buffers very fast (overrun). Some chips cannot cope with small buffer sizes, so you have to increase the buffer length to ease the work done by the sound chip.
In my experience, you'll get a couple just from booting up Mixbus...then it's fine. I have read that the 48khz setting helps.
Mixbus has a preference where it will list the xruns (up top by the dsp readings). a "shift click will zero them out" so you can see if your getting xruns after the initial bootup.
I'll have to have Mixbus in front of me to answer anything else.