- What are the precise settings you use in the export dialogue? (post a screenshot if possible).
- What application do you use to audition the file after export?
- Are you exporting the entire mix from the stereo bus of Nuendo? (if so what is the level of your stereo bus faders at the time of the export?).
- Do you import the file into the project after export?
- Are you using the exact same routing of the audio signal when you audition the file after export?
AFAIK there are no level problems with export in nuendo. Differences are usually a problem with the signal path or confusion when monitoring the signal after export.
For more ideas, see also: