I didn't pay much attention to impedance plots on ribbons yet because they are "resistive". Sure but the little resonances that plague planar magnetic drivers DO show up in an impedance plot. Below is an example. Other than confirming that these ribbons behave almost purely resistive, the little bumps tell us more.
The first "resonance" of a RD75 is a wide peak with center frequency around 125 Hz. The first resonance peak of the Carver 60" ribbon is a well defined bump and centered around 185 Hz. These numbers and peak shapes confirm once more that the tensioning between the two drivers is different. There are two more impedance peaks visible in both drivers with a similar pattern. Wider peaks at lower frequency for the RD-75 as compared to the Carver 60" ribbon.
The location of the peaks do correspond with frequencies where harmonic distortion peaks. The wider peaks on the B&G RD-75 seem to correlate with the "smearing" out of the harmonic distortion over a wider range. (See the distortion plots earlier on this page.)