In my wire investigations I have learned that 6-9s wire is nearly extinct and even 5-9s is practically unaffordable. As the world changes toward miniaturization, highly complex component circuitry and wireless data and signal transmission, wire is less critical. It seems that some of the best sources (both audio and aerospace) are using 4-9s with careful casting, drawing, gauging and coating technologies. To your question, Thiel maintained a steadfast commitment to using the best wire availableHi TomI probably already shared this in a PM, but I found the Cardas hookup wire and binding posts to make a really nice improvement in SQ. In fact, I would rank the improvement as tied with the Mills resistors for second biggest improvement after replacing all caps and coils. I hope you are able to audition Cardas as you work on the R upgrades.
That said, my experience aligns with with Prof in that $$$ AC cabling does not yield a corresponding improvement in SQ. As always, YMMV.