What I am talking about is quite obvious when listening to this comparison by the reseller of Ruud's circuit (Vintage U67 vs. Ruud67 ) - knowing the Ruud67 is equipped with a chinese K67 in this case and might sound different with a proper capsule (I am using Arienne's K87):
The difference made by the microphone capsule is more obvious than any other accessory. I found that there are differences between many of Neumann's original K67s, but they are far less obvious than other capsules. Perhaps these subtle differences I hear are the result of natural aging over the years.