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 one is interesting. ECL113 was developed as an audio amplifier for TV sets and curves would suggest excellent linearity, but measurements show a very harmonic rich content. Which is strange and could be due to damaged tube (only one in my stock). Either way, it shows one more time why curves are just a part of the story and spectrum is what counts. Available for sale.