It's difficult to troubleshoot a problem like that without being able to hear what you're hearing, or without a more detailed description of both the setup within MainStage [the Concert] and your software/hardware setup. I could guess that somehow the audio signal is being doubled up and there's a slight delay on one signal path, which would result in a static flange [a sharp nasal tonal variation that I suppose might be described as "phasey"], but from the minimal description there's no obvious way to know what might be causing the "phasiness". If you have an interface, maybe your interface's software is involved..?
Some things to look at..
- Are you sure there's no effect strapped on the main stereo bus in MainStage?
- If you play audio from other applications on that computer does it also sound phasey?
- Do you have an audio interface, and if so does it have its own routing/monitoring software that might be adding an effect?
- Do you have any kind of internal routing software installed, like Soundflower or Loopback?
- Are you monitoring through the computer's built-in speakers or headphone out, or through external speakers?
- Whichever you're using for monitoring [as above], what happens if you switch -- still phasey?
- Did you try a hard computer shutdown/restart -- if there was a software glitch that sometimes can fix it..?
I've attached a basic MainStage course -- check out video #3 on setup, maybe something in there might be helpful..