Geoff Hill's Tetrahedral Test System?

Does anyone have any experience with Geoff Hill's Tetrahedral Test System?
Taking reliable and repeatable measurements of drivers (especially at low frequency) has always been a challenge.
This Tetrahedral Test System promises to solve those issue without having to go to an anechoic chamber.
Has anyone here made their own? They seem relatively easy to make.
The thing is, I'm more interested in harmonic distortion measurements than frequency response. Currently, I'm using Steps from Arta software. The issue is with steps you can't do gated measurements, so room reflections affect the measurement. If I place the mic near field (to minimize effect of the room), I'm afraid the SPL level is too high for the mic to handle and the distortion introduced by the mic corrupts my measurements.
At this point the solution seems to be to find a mic that can handle 110 ~ 120dB level without distorting.