What kind of tub system are you using and how are they set up? Include things such as heat source, size of water bowl, substrate, etc? Finally, what is your current humidity at?
I have had my balls and boas in all types of setups, from Rubbermaids, Freedom Breeder racks, and 4' enclosures and have been able to adjust them from proper humidity 70%+ in every instance using newspaper sheets as substrate. My first thoughts would be to make sure the water bowl is large enough (on the cool side or middle of the enclosure), perhaps reduce the amount of ventilation (screen tops are bad), and make sure to use belly heat set at the appropriate temp on the hotspot of about 91 F in the enclosure and set up in a room that has an ambient temp in the mid 70's. More details about your setup will help with the specifics.
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