There seems to be a question about how difficult it is to take an accurate close mic measurement.
Just for fun, I tried a few close mic measurements of my NHT 2.9 speakers. This speaker is a 4-way speaker with a 10" sealed driver that uses a passive crossover at 100 Hz. For the measurement microphone I used my RS Digital SPL meter set to the 100 dB C scale, and applied straight standard SPL C scale compensation factors. Close mic sweeps.
SPL level was calibrated for REW and TrueRTA. REW used 250Hz as the upper sweep level.
My room has little room gain.
Sweeps vary below 13 Hz because of room gain and whatever.
General conclusion. It is difficult to measure FR below 15 Hz in my room (little room gain). Also, the Stereophile reference varies from my close mic measurements around 70 to 100 Hz.
I suspect that room gain in MKT's room shows up in his close mic measurement. Then again, he may be doing something else that I do not have any knowledge about.
Top curve is average of 5 sweeps, smoothed to 1/6 octave.
Middle curves are 5 sweeps overlaid. I took more sweeps, but tossed out the ones that looked real jagged below 15 Hz.
Bottom two curves shows Pink Noise with 100 averages compared with the top curve (averaged smoothed sweep). Same results.
9 sweeps. Variations at frequencies below 13 Hz.