We have a 2-handled tub faucet (Kohler) that is leaking. The plumber I called for an estimate wants to change out everything -the faucet, the handles, the shower, and do it by cutting a hole in the wall behind the tub. He says the 1976 plumbing should be replaced. I just want the tub to stop leaking - can we just replace the handles? What should I look for to justify replaceing it all?