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.
Very linear tube with a clean spectrum! Notice how little THD changes from bias current, really remarkable. Developed from EL42 for lower heater consumption and intended for car stereo marked (never made it there). Should be good for a very sweet 2W amp in triode mode. Available for sale.