When using the AU plug-in version of an iZotope product in Studio One 6.5.1, no audio signal is passing through the plug-in and you cannot control any parameters.
This is a known issue with iZotope AU plugins in Studio One 6.5.1.
Presonus are aware of this issue. The issue has already been fixed and the solution will be released with a upcoming maintenance update in early 2024.
1. Especially for new projects, we recommend using the VST3 plug-in version, which is not affected.
2. If you have existing sessions with various AU plug-in instances, we recommend reverting to Studio One 6.5.0 to continue using the iZotope AU plug-ins until the issues is resolved.
If you want to downgrade to version 6.5.0, please contact Presonus support in order to request the installer.