My pair of Polks is labeled RTA-15T on the back of the cabinet. Is this the same as a RTA-15TL? I can't find reference to a RTA-15T anywhere. Along the same lines, I see RTA-11T and RTA-8T and a RTA-11TL and a RTA-8TL. What makes them different from each other that wouldn't be the case for a "15" series?