I know I’ve read about fillers that are used to smooth a rough subfloor. What types and/or brands do you recommend?
You can only fill a concrete subfloor, with special compounds made just for this purpose. Check out http://www.quikrete.com, they have been in the concrete patching business for many years now. Stay away from any gypsum based compounds. They will fail.
But if it’s a wooden subfloor (you didn’t say) I’m not sure what you mean by rough. Please tell me what it is made of and how thick it is and what kind of hardwood floor you intend to install on top of it.