Seeing as how there is no direct flight from BNA to MAD, the routing is important.
Regardless, just do a search for Premium Economy class as any of those (particularly on American) are going to be quite new. As a passenger the age of the plane doesn't matter, what you mostly notice is the age of the interior which can be refreshed. Note that premium economy is a completely separate class compared to the Economy+ packages the US airlines provide.
Regardless, the American planes flying this route are generally updated interiors.