If I'm reading your results correctly, groundplane max and min are 89.5 and 83.5, i.e., +/- 3 dB, better than any results I've ever obtained measuring LF conventionally. The technique is credited to Mark Gander at JBL in Stereophile 28:1, January, 2005, P. 68.

I'd call it most excellent work, Old_4312. May we have a summary of your findings, then, regarding LF measurements? Some readers may not get what's going on otherwise.

Also, tell us about the measurement software you're using, please.

Tip: See D'Appolito Chapter 4 regarding loudspeaker measurement microphones.