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Hi Sylvain!
Can you upload your file to your Dropbox or Google Drive and share the link?
In general, your approach makes sense to me, but I don't really understand why your Brainamp has an average reference electrode. This doesn't make sense, because an average reference is by definition the average of all involved channels, which means you cannot have a dedicated electrode for this. Maybe the data are collected with a FCz reference, and then they are re-referenced to average afterwards?
Anyway, I think the actual file would help us track down your issue.
Clemens