Sounds like you might already be leaning this way, but the most cost effective solution will be to drill another row of holes in the tubs you already have. A smaller water bowl will help, and you might think about switching your substrate to paper towels, newsprint or kraft paper, since they don't really hold humidity unless you mist them.

If you switch over to 28 qt tubs, and keep the same set up you have, you will see a drop in the humidity (larger air space), but the 28 qt tubs most likely won't fit into the same place you have your 15 qt tubs now.