Hardwood underlayment for floating floor over concrete?

Does anyone have advice on what (if any) underlayment to use beneath hardwood flooring (KD Birch) when the subfloor is 2 layers of advantech floating over foam, poly vapor barrier,… Does anyone have advice on what (if any) underlayment to use beneath hardwood flooring (KD Birch) when the subfloor is 2 layers of advantech floating over foam, poly vapor barrier,…Read MoreGreenBuildingAdvisor