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.
Special long life (E – 5000h) and vibration proof (V) version of 6Z38P. Very low (for soviet tubes) parameters spread and good THD consistency (as can be seen from histogram). Available for sale.