Am about to lay a solid oak floor (wide planks) onto an existing wooden floor (original floorboards)in a first floor flat and in desperate need of clarification as to what, if any, underlay to use. Been hearing different things with some saying solid wood should always be nailed down without underlay to original floorboards and other people recommending to put plywood under it first, whilst still others recommend soundproofing with underlay. Could someone please clarify?? Thanks so much