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.
This is my personal find 🙂 Extremely linear soviet pentode that stayed under the radar all these years, until now that is. Should be equivalent to western 6BH6, EF190. Very low noise, not microphonic and has really clean spectrum. Seems to have a couple of sweet spots at around 160V and 210V anode voltage. Push as much current as you can get away with, but watch out second grid dissipation, as it will not take much abuse (0.3W).