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.
Don’t be fooled by a very low noise floor – this is a really clean spectrum! H2 content is quite substantial, but with active load like CCS or μ-follower this tube sings pretty sweet! Give it a try in a low-gain application like voltage amplification stage for hybrid amp and you will be surprised. Available for sale.