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.
Another miniature version of almost cult-like 6SN7GTB. This tube was a follow-up to 6CG7, but without intersection shield. Less linear than its predecessors (cost cutting) but really clean spectrum at optimum BIAS point with nice monotonic harmonic profile. Sounds sweet too!