Well, I have entirely given up on humidifiers. What used to be a reliable brand, Bionaire, was bought up and ruined. Anything they put out now requires a ridiculous amount of maintenance and the constant purchasing of additional products to keep it from getting moldy or otherwise contaminated.
My solution has been to use a vaporizer. They require minimal maintenance, and in a large high-ceilinged room like my studio, there is no hazard from warm vapor. After a couple of days, it has raised the humidity enough to help.
Sponges get dirty, so I would prefer to hang up wet towels and let them dry.