This test to acknowledge my assumption that the closed box would dampen the ribbon in the extended low in such a way that it would lower the harmonic distortion as previously seen in the ribbon only plots. The microphone was placed near-field at 1.5" from the ribbon.
As expected, the harmonic distortion in the extended low is down. The distortion from 300 Hz on remains pretty much the same as compared to previous dipole measurements. The distortion is primarily third harmonic. The region 300 Hz to 2 kHz shows mild resonances as witnessed by the distortion pattern and the "peaking" in this area in the magnitude plots at normal listening distances.