the polyurethane is creating a barrier that is prohibiting any cleaner to actually add life and luster to the wood. Its like clear coat on a car.
Poly has a bad habit of aging and turning orange or yellowish. Nothing will work at all until you remove all the poly. this can be done with sand paper or a paint remover.
I was going to quickly suggest the Formby's Products for wood restoration, but the poly nixed that for sure.
Nothing will work until the poly is removed and then you can add all sorts of life to the wood and then not go back with poly but consider tounge oil or boiled linseed oil. Then follow up with orange oil or lemon oil for luster.
Sorry about the non answer, but keep us in your back pocket.