Based on everything I have read there was a great deal of "black art" involved in mfg vacuum tubes. A great deal of this information has died with the people involved in the business. Remember these tubes were made long before ISO 9000 was around so not every detail was written down.
There are also issues related material and toxic mfg processes which when tied to todays very low production volumes are factors to consider. Lastly many valves were also produced in Mil Spec flavors which were real quality sensitive customers.
To my ears, in my gear, in some applications nothing current beats NOS in particular when used in line stage applications. Power Tubes i.e. KT88 and others may not be as fussy.
It comes down to your ears in the end.
There are also issues related material and toxic mfg processes which when tied to todays very low production volumes are factors to consider. Lastly many valves were also produced in Mil Spec flavors which were real quality sensitive customers.
To my ears, in my gear, in some applications nothing current beats NOS in particular when used in line stage applications. Power Tubes i.e. KT88 and others may not be as fussy.
It comes down to your ears in the end.