Humidity almost always rises at night. Some mumbo jumbo about dew points or absolute/relative humidity or something. Ask Tom Skilling on WGN- he'll over-explain it for you.
You're wise using "good water" in the humidifiers. The manual says you can use tap water because they want you to wreck the unit and buy a new one.
Your issue is that your volume of air you're trying to humidify is too large. Just give in and give your den or spare bedroom to the orchids. Take out all the furniture, and go NUTS. Garages and basements work too!
You also need to get lots more plants and group them together to help localize some of the humidity- seriously! They "breath out" moisture through their leaves (transpiration). Sorta like having a bunch of people in an elevator.
You should see NYEric's collection (he has a crazy thread here). No one knows where he sleeps, sits, cooks, or eats because every surface of his apartment is COVERED with plants! Resistance is futile!
Anyway, keep it up. I think you know more than you're giving yourself credit for.