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.
Well known pentode, mostly used in phono pre-amps or as a 300B driver. Very low noise, not microphonic and really good sounding tube. Current prices that approaches stratosphere well confirms that. Sweet spot seems to be between -2 and-3V grid BIAS and as much current as dissipation allows.