As alluded to above via driver size, air-coupling is a factor in the experience of bass authority. The 3.7 10" woofer has 50% greater area to drive its wave-front, reducing compressive non-linearities of air. Bigger makes cleaner. The 3.7 cabinet is also more inert and its baffle is less compressive for attack transient integrity. Regarding prof's thought about bass voicing, the 3.7 was Jim's work, the 2.7 incorporated various outside engineering input. Jim tuned his bass alignment to Q.707. Many designers give a little slop for "bloom, bigness, warmth", etc. I heard the two speakers at Thiel at finalization and heard the 2.7 bass as tuned a little looser. Most designers try to second-guess popularity, expectations and so forth. Jim was fairly immune to those ways.
Also note that image is highly dependent on cabinet and driver edge effects. Prof mentioned the driver part. The 3.7 cabinet design (love it or not) is very highly functional regarding diffraction, even though the 2.7 might qualify for world-class, it is not as good.
A factor that contributes to "great bass" is that of articulation which, in a first-order design, includes the low end of the mid-range driver. Jim's 3.7 XO treatment is much more sophisticated and uses better parts than the 2.7 due to budget constraints and designer choices. The 2.7 midrange is fed through a 400uF electrolytic cap, albeit with a PP and styrene bypass in parallel. The 3.7 uses a bank of 75uF polypropylene caps with a styrene bypass. Multiple smaller PP caps provide faster reactions and less distortion than a large electrolytic.
Unsound, the 3.5 equalizer addresses only the bass with a simple, shaped boost centered at 22Hz or 40Hz depending on selection. Our reference set-up during development was bi-amped with identical amplifiers and 4 identical wire runs providing no EQ pollution into the midrange-tweeter circuit. (We were subsequently amazed by how many ways users could screw things up with varying amp and cable configurations working at cross purposes. So the bi-wiring option went away.) The EQ circuitry is elegant enough, but the budget required utilitarian execution (and use of generally inferior interconnects) adding some grain and haze to the signal. Jim considered the ported solution (O2, O4, CS2) to be inferior to sealed-box bass and only grudgingly accepted the market necessity of the passive radiator rather than the equalized bass in aspiring products. We aired the possibility of an EQ for the CS5 (the CS5 followed the equalized 3.5) and we talked about a follow-up super edition with an equalized bass. But Thiel was a one-man development lab experiencing high growth, and there was not time to explore such niceties.
One intriguing reincarnation for a CS5 Super would be to add balanced equalizers to the CS5 bass driving a separate bass input. That bass section has three woofers in two configurations loaded by two sub-enclosures. All bass frequencies up to 500 or so are covered by that subsystem.
