If it's possible for you you could always switch to paper and then bump the humidity up in the room rather than just the tubs. You can get a humidifier or damp towels on the radiators and what not. There's always a way lol
Best substrate I've ever used is bark chips but I couldn't use it in my tubs because even when I'd dry it for a while before using it it would still be to humid in the tubs as where I live is pretty good for humidity. . If it was lower it would have been perfect. I use bark chips in my wooden vivs tho and its definitely the best substrate I've used and i wouldn't change it.

Good Luck