Similar though to a reflex cabinet, the speaker will continue to go low and chances are, will kill itself in the process. All the 4560's I ever used had something else under them with an active crossover so I never saw this sort of thing. I was thinking it was the Fs of the E145 because it would be a big coincidence if the room resonated at the same point. Maybe but the Mic close and run the test again.
Allan.