Do you mean that the port output is out of phase with the woofers? I guess that would be the only way for the on-axis response to be lower than the port output in the region from 25 to 48Hz.
I don't know exactly what he meant, but at the port resonance frequency, the port output will always be very, very close in phase with the driver. Moving lower in frequency, below the port resonance, the port and the driver will be increasingly out of phase, such that the driver excursion goes wild even though the net sound pressure in the far field drops in accordance with the phase-driven cancellation. The impedance graphs indicate that the port resonance is between 25 Hz and 30 Hz. Thus, if you were talking about the on-axis response being weaker than the port output below maybe 20 Hz, this would not be mysterious. It would however be mysterious if this were the case in the region you mention, between 25 Hz and 48 Hz. I had previously read your other post:
... The on-axis measurement shows a very smooth, shallow roll-off beginning at about 80Hz that looks much more like the roll-off of an acoustic suspension speaker than the sharp knee of a bass reflex. Looking at the graph labeled "Revel 226Be On-Axis Response vs Components' Nearfield Response", the port has a pronounced peak at 40Hz which appears to make no contribution whatsoever to the on-axis response...
The specific graph you mention here is a bit treacherous, because Erin cannot open up the speaker and disconnect all the drivers but one. And with the port output it is even more difficult, because if you disconnect the driver, you lose the port output along with the driver. So in this graph, the individual outputs as they appear in the graph are only approximations to the true individual outputs. Amir produces the same kind of graph and the same limitation applies to his graph as well. Potentially they could plug the port to obtain a purer graph for the woofers, but this wouldn't correspond correctly the woofer's behavior when the port isn't plugged. And when you consider that at these low frequencies sound is omnidirectional, there isn't any practical way to get a clean measurement of either the port output or the woofer output, where each would be unaffected by the other. The peak output of the port should be at its resonance, which we know from the impedance measurement is located between 25 Hz and 30 Hz. Yet, as you state, the graph indicates that the peak is just a hair below 40 Hz. I don't think there can be much doubt that this is caused by the mic picking up the sound from the woofer, the primary effect of which would be to shift the apparent response of the port to frequency slightly higher than it actually is.
You also said that the port's output appears to make no contribution to the on-axis response. When I looked at the graph it was not obvious to me why you had made this inference, but no matter the exact reason, the explanation is most likely the same, i.e., the individual curves you see in that graph are rough approximations where each of them is affected to some extent by the others, especially at low frequency. If you are able in your head to work out what corrections would need to be made to the individual graphs such that they would sum in agreement with the overall speaker output, then this mental image will probably be a more correct image of the individual outputs.
One thing that I very much like and am surprised by, which you pointed out, is that the net output of the speaker closely mimics the smooth, slow rolloff of a sealed speaker. What this suggests to me is that the Revel engineers were shrewd enough to figure out how to use the port to improve the efficiency and sensitivity of the speaker in the deep audible bass without suffering the usual drawbacks, or at least without one major drawback. This looks to be an amazing speaker, and I would very much like to hear it at some point. Anyone who doesn't believe that measurements reveal the sound quality of a speaker, and who would doubt that this speaker sounds as good as the measurements indicate, would surely experience an awakening of sorts upon listening to this speaker. I still can't believe that Erin sent them back. What was he thinking?