I want to tile our bathroom floor. It's an old house and we currently have a bad hardwood floor in there (pet-stained by previous owners and not worth saving). Can I install a thin subfloor (Ditra?) right on top of the hardwood and then tile? Or should I remove the hardwood, install a subfloor, and then tile? If I remove the hardwood, what should I use for the subfloor? Thanks much!