Hi timbobroberts,
We know of no compatibility issue with Mavericks. I assume you're using the 2.1.3 version? You can check by mousing over the word "Eventide" in the GUI, the version number will pop over.
The behavior you're talking about in AU sounds like it's loading the mono rather than the stereo version of the plug-in. We don't test internally in Reaper, so I'm not very familiar with it, but IIRC there is some sort of channel routing matrix for stereo plug-ins in reaper. How is this set?
As far as the VST version is concerned, there is a mix control in Omnipressor, though it's only accessible as an automation parameter. Can you check automation to see how this is set?
Dan
