This mode corresponds to one half of the wavelength that will fit into the width of the room (15 feet in this case). It is calculated by ½ * speed of sound / Width = Hz. In this case it is 37.6 Hz, which is exactly the same as the 2L mode. This is going to create a huge bump in this frequency, which will be difficult to compensate for.

If room modes are a concern, you may want to learn more about the PARC. A Parametric Adaptive Room Compensation system developed by Rives Audio to nearly eliminate bass mode problems.