Hello mthomas,
three PB16s in one room! Good lord, that is a beast of a system! To answer some of your questions:
I tried to emulate Josh's measurement methodology as closely as possible, since he really sets the gold standard for subwoofer measurements and reviews. He has given me a ton of advice and assistance in this, and, as a result, my own measurements of a same model sub that he measured average less than a one dB difference. Josh is a great guy and has been tremendously helpful!
As for the measurement as 12.5 Hz, what tells part of the story of what is occurring can be gleaned in
this image in the 12.5 Hz panel. The third harmonic is a very heavy presence at both 10 and 12.5 Hz at all but the lowest drive levels seen there. I can only speculate about the cause. First we have to keep in mind that 12.5 Hz is below the intended range of operation for the PB16 in its ported modes, so trying to get monster 12.5 Hz output from it is, in a sense, misusing it. 12.5 Hz is well below tuning, I am not sure what we should really expect from that point except for port noise. The driver is only being controlled by the limiter at that point since there is no more backspring pressure, so it is no wonder there is a lot of distortion. That being said, the other ported subwoofers I have tested did have a greater ratio of fundamental-to-distortion at these super low frequencies. The only other subwoofer I have seen with this constant presence of odd-order harmonic distortion in these low frequencies was the Hsu ULS-15 mk1, which also used a underhung driver, so it may have something to do with the way underhung topology reacts to these extreme deep frequencies? That is just wild speculation though.
Something we should keep in mind is when we talk about hearing deep bass such as 12.5 Hz, what is it that we think we are hearing? We need
over 100 dB just to sense 12 Hz at all! And even then we can not discern pitch at such deep frequencies, and sound at these frequencies is just perceived as discontinuous noise. If you blast a 12.5 Hz tone on either the PB13 or PB16, you will definitely hear something, but that something is not 12.5 Hz, it is the harmonics of 12.5 Hz and perhaps port turbulence.
Something else to note about Ricci's results with the PB13 is that while he could only measure a passing burst at 89 dB at 12.5 Hz, he could achieve 97.4 dB without any distortion thresholds. From this, I would guess he was seeing the same kind of behavior that I was; lots of 3rd harmonic distortion even at modestly high drive levels, as I speculated before, perhaps an attribute of underhung topologies since the PB13 uses an underhung motor as well. Similarly, disregarding thresholds, I was able to get a 103.0 dB peak reading (100 dB RMS), so the PB16 is definitely capable of moving more air.
The short version is I think what you are experiencing is overall greater sound pressure levels in really deep bass frequencies, but not the fundamentals so much. Even the third harmonic of 12.5 Hz (37.5 Hz) is still a very deep tone and is certainly in a range where human pitch discrimination is not great. Hopefully that gives you a little bit of insight on what you are experiencing.