Hauptwerk 4 doesn't supply reverberation, which is in the Caen Surround samples, so this is a question for Sonus Paradisi. Contact through their web site.
Are you sure all the release samples are loaded for that stop, both front and rear, with full releases, no truncation?
You might need to upgrade to a current version of Caen. But the current Caen 2.67 requires Hauptwerk 4.2 or higher, so you'd have to upgrade HW to 7 first.