ToTo Man
the band not the dog
The distortion measurements should already be there. In the main window of REW you should see the measurements you took as a list of tabs on the left. Then to the right there should be a row of buttons, SPL & Phase, All SPL, Distortion, etc. If you click Distortion you should see a graph with the Frequency Response on top and the Distortion measurement below. Make sure all of the harmonic checkboxes are ticked apart from Noise Floor. Also you may need to change the Y-axis scale, I recommend 90dB as the upper limit and 0dB as the lower limit.ToToman.. interesting to note that the HF units are often mismatched.. I understand the pepperpot machining also plays into this. Will also do some distortion measurements as you suggest.
PS - To give you an idea of typical frequency response variations between Tannoy drivers, here are some I've measured over the years:
This one would have been a good candidate for HF re-alignment: