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.
TV line output pentode, successor of PL36 (=EL500 with 27V heaters). In triode mode linearity is excellent, but spectrum suffers from high level of uneven harmonics (5th particularly) that you can’t get rid of no matter what operating point you choose. This is common to all line output pentodes I have measured and could be culprit of “harsh” sound reported by a lot of users. Tube needs substantial current (<40ma) to ge into linear region. Not microphonic and much better behaved (oscillation wise) then PL36.