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 6BK7/6BQ7. Sometimes falsely advertised as equivalent E88CC (to push the sales) which is not true. Curves are quite different so operating point will be sub-optimal when interchanging. Very linear and good sounding triode but has same problems as all soviet tubes – large quality and parameter variance. This particulate example always has traces of higher harmonics (below -110dB). Other factory/year – can have same clean spectrum as premium Siemens E88CC, but for a 1/100 of a price. Available for sale.