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I'm not sure if I can offer a solution, but I'd like ti weigh in with a couple of observations:
The Aprilaire 400 is the smallest bypass type humidifier that aprilaire makes and is suitable for homes up to 4000 square feet - The model 800 is a steam humidifier that should be suitable for homes up to 6200 square feet so the humidity output is much higher.
The installation of the 800 is pretty specific - The dispersion tube has to be located in a straight section of duct upstream of any bends or obstructions, (definitely not in the return side) the unit has to be level and the tubing has to have slope away from the dispersion point - All of this is to keep the water from condensing and dripping into the ductwork.
It's possible that the dispersion point is located too close to the unit or in an air current eddy, causing condensation (humidity not being carried down the duct with the airstream) - It's also quite possible that the unit is producing too much humidity for the airflow - The 800 can be configured with a downstream humidity sensor that acts as a high humidity limit to keep the supply air from over-humidifying.
These are the installation guidelines that explain some of the details :ftp://ftp.aprilaire.com/Fulfillment/Humidifiers/800-Series-Humidifier-Install-Instructions.pdf