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.
What a surprise! I would never believe this was a tube if somebody just showed me the THD spectrum… OK, maybe not a fair comparison to other Indirectly Heated Tubes as this is only DHT pentode in my stash, but still, what an impressive result! This is a Soviet copy of Wehrmacht RL2/4P6 tubes and probably last DHT pentode ever made. Couple sweet spots can be found at anode voltages of 160V and 240V – ideal for pre-amp and amp operating points. This tube definitely needs more investigation.