My statement about the CS5 vs 5i woofers was misleading. I did mean that the 3 woofers were different between the 5 vs the 5i. But I did not mean that the physical mass loading was the difference. That is merely the evidence by which to distinguish which version you're seeing.
The MDF and rubber mats act the same, governed by their mass to load the upper and lower subwoofers which cross out at 50Hz (-3dB). The third, middle woofer crosses out at 400Hz. All 3 contribute to the low end to create a system -3dB point below 20Hz. -1dB @ 23Hz with a gentle 12dB/octave extension below that. This bass alignment is highly unusual and innovative. I believe it could have defined Thiel's future if the entire speaker had maintained a higher impedance. As it is, the impedance magnitude falls below 4ohms at 3kHz and steadily drops to below 2ohms @20Hz - although its curve is smooth and non-reactive it is nonetheless punishingly low and requires brute-force high current delivery from its amp.
Those 3 woofers are two separate circuits with 2 separate enclosures. The mid-woofer with its lighter cone mass produces the bass transients. The pair of sub-woofers (below 50Hz) are physically tuned via that additional mass to both roll off at 50Hz as well as to attenuate sub-bass frequencies. That physical shaping allows a very simple XO circuit of an inductor to help roll off its top end plus a zobel network to control its impedance. Note the absence of the usual bass impedance humps in the stereophile curve.
The actual upgrade between the original 5 vs the later 5i drivers is the more sophisticated motor with focused magnetic field and copper pole and voice coil shunts. The sonic improvement is considerable resulting in greater clarity with considerably lower distortion.
If that 5i system could be implemented with higher impedance across the board (it could be), then a wide variety of amplifiers could drive it without audible degradation due to current-delivery demands beyond their capability. In the day I lobbied for such in a model 5.2, which never came to be.
