Solid Hardwood, Engineered and Laminate Flooring - Impossible? laminate on a VERY unlevel/unflat floor?

I'm fairly certain unflat is not a real word, maybe even unlevel...? So far everyone locally has said it is impossible to make my floor level or flat enough for laminate. The flooring is simply 2x6 tongue and groove, sitting on round rough-hewn beams. This is in the upstairs (kitchen, livingroom etc) and the result is a floor that dips and rises, like the ocean but no where near as soothing. With a 6' metal yard stick on end I have at least 1/2" gaps in many areas. I've thought of trying to heavily sand the high spots, fill the depressions, and then screw down 3/4" plywood. Is this correct? Is there a special leveling product I should use on tongue and groove? Will the leveler crumble after fastening and traffic? It seems doable. My wife thinks I'm insane, I wouldn't know.