You will have still have a number of common modes shared with a rectangular room, just more of them, you will have your common height mode, both width modes and your length modes, however, depending on the placement of your sub, the second width/height modes may take little affect, that covers your axial modes at least. The tangential modes would also be similar to a rectangular room but your oblique modes would almost for sure be either eliminated or at least different.
Also due to the dual axial modes, you may get some cancellation, not perfect, but decent, this would in help to smooth out your response.
