What's the square footage of your house? What are your outside temperatures?
The Aprilaire 400a has the same output as the 600a but uses less water. The nice thing about this type of humidifier vs. the drum type is that you should not have to worry about junk growing in your humidifier, but you are supposed to replace the water panel for the flow through type once a year. It sounds like the 400a uses a wick type water panel instead of a non-absorbant type.