At first glance, coils are 4-9s, best of form. Significant improvement could be made via cap and resistor upgrades from AXON, electrolytic and sandcasts.The coils are certainly of higher quality construction than those on my FST-sourced 2.4 boards. And the yellow caps appear to be the high quality polystyrene 1 uF that Thiel commonly used. I see one bank of electrolytic caps. Those are probably shunts but I found shunt cap upgrades to be audible. ELs not only drift with age but they are generally lower SQ than film caps. The other caps are probably metalized film type but it’s hard to tell for sure. I see at least one cap with the Solen logo. The resistors appear to be the sandcast wirewounds.
Beetle might have an opinion.
I am confident that improved SQ can be had by upgrading the parts quality. If that was my CS6, I would replace all resistors with Mills MRA-12s. I would want to see a schematic before commenting about specific caps but I can say that better caps substantially contribute to improved transparency and resolution. Whether this would fully mitigate JA’s one criticism of the CS6 I have no idea, but it would certainly get you at least part way there. Andy2, curious to know what you see on that board. How did you surmise an all-pass filter? An all-pass filter suppresses no frequencies, right? Seems like such a "filter" would invariably allow the driver to excite its resonant modes.
One thing I learned form Tom - and that greatly surprised me - was that Thiel used the same level of parts quality on all models from the 1.X to 7.X. I confidently predict that better parts quality will improve SQ for all models. That said, DIYers should know that it’s not necessarily as simple as replacing OEM parts for higher quality ones with "identical" values. In particular, changes n DC resistance can modify the voicing.