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.
Soviet copy of E88CC. Very linear and good sounding triode but has same problems as all soviet tubes – large quality and parameter variance. This particulate “Reflector” example from the ’62 has a really clean spectrum, same as Siemens E88CC, but for a 1/100 of a price. Other factory/year – can have traces of higher harmonics that you can’t get rid off no matter what operating point you choose. Another difference is absence of grid current at low BIAS voltages which is an advantage compared to E88CC.