I've been using Siemens CCa's(from the late 50's-early 60's) for a number of years(six in my CDP, lately), because of their transparency, frequency extension, bass accuracy and sound stage abilities. The CCa was a specially selected 6922 family tube, chosen for low noise, long life, low microphonics/transparency for the German telephone, telex, and telegraph services(VERY picky). Many of the German telephone hubs used tube equipment and for telephone equipment you want a "noise/coloration-free" environment. Very much worth the effort to find, and every penny spent, for the sonic rewards that they present.
Telefunken, Lorenz, Valvo and Phillips all had selections for the "German and Dutch Post" also.