OK, just can't stop playing with REW.
I stood the sub up and moved it into the corner of the room to see how that would affect FR. Previous measurements were with the sub "horiztonal" and beneath the screen (as pictured earlier in this thread).
Moving it into the corner made a big difference. The graph below was done with the mic in the exact same position as the first graph I did with the big dip at 58.5 Hz, but there is no trace of that dip now.
Also note I changed the scale to 5 dB increments instead of 10.