The pre-fab bases require a flat surface. If the existing tile was a floor, and not an old shower, then it should be flat. It also requires level, so check that. If it was an old shower you are hoping to reline, not a good choice.
While you could probably put a marble base directly on plywood, it would probably be better if it was over cbu. Thinset or a medium bed mortar would be the best choice. You want all the minor imperfections filled in so it is 100% supported, and gluing it down isn't likely to do that. Mortar will.