To be brief. I found the CS6 to be overly warm with emphasis given to the 2nd octave, to the point that the bass can be obnoxious and distracting. IMO the bass detracts from speakers clarity. The midrange still has the same Thiel sound but there is less of it with these speakers. Highs are rolled off slightly so the speaker lacks air and extension of the CS2.3.
The CS2.3 (I have the pre-upgrade midrange/tweeter) has excellent extension up top with wonderful midrange transparency and slightly lean bass. I'm told that the midrange/tweeter upgrade produces a laidback sound, but I have not heard it for myself, so I will defer to those with first-hand experience. IMO the only thing the CS6 has that the CS2.3 lacks is about 10hz of bass extension.
The CS2.3 (I have the pre-upgrade midrange/tweeter) has excellent extension up top with wonderful midrange transparency and slightly lean bass. I'm told that the midrange/tweeter upgrade produces a laidback sound, but I have not heard it for myself, so I will defer to those with first-hand experience. IMO the only thing the CS6 has that the CS2.3 lacks is about 10hz of bass extension.