Do you have holes drilled into the tub? I ask because, in my experience, tubs have more of a problem with retaining too much humidity, as opposed to causing low humidity. I use newspaper substrate myself, but you can crumple a few pieces (if you use it) and spray them down nicely with water.. that should help. Of course you do not want a sopping wet bottom substrate..
During shedding time, I really raise the humidity in my tubs, to the point that there's condensation on the sides of them.