All tube operating points below are by no means definitive good sound recipe for your particulate tube project. Lowest THD point is not necessary best sounding one and there is always some variance even in a NOS tube batch from the same manufacturer! Below information is only useful as a general starting point and is ment to get you in a ballpark when no measurement equipment is available.
Interesting pentode with clean spectrum in triode connection. Used in famous Telefunken V76 mic preamp as output valve (in pentode mode, choke loaded and wrapped around in local feedback loop together with EF804). Strange choice as these tubes are very microphonic (at least Siemens examples I have).