Above is the measured frequency response of the system, stuffed as indicated above. While the low frequency response remains approximately the same, the upper frequency response has changed significantly. The peka and notch between about 120 Hz and 200 Hz has basically disappeared. Again, as I'm driving the tapped pipe subwoofer using the subwoofer channel on my amplifier, the response also includes the effects of an active 12dB low-pass filter @ 100 Hz. However, the response above 100 Hz is significantly smoother in character than the response of the unstuffed pipe.
Above is the frequency response of the stuffed system compared to the raw system. The smoothing of up the upper frequency response is readily apparent here. And it looks like I got the driver seal right this time - the low end response has apparently improved.
Above is the impedance response of the stuffed system. The peaks and dips are a bit lower thanthe HornResp predictions (the green line) now, and the magnitudes are significantly different - not unexpected, as the damping would cause that.
The graph above compares the impedance response of the stuffed system (red) to the unstuffed system (blue). The peaks and dips have clearly shifted down a bit. This suggests that the resonant frequencies of the system have been shifted down by the addition of the stuffing.