Why aren't new vacuum tubes as good as old ones?

Can anyone provide data or evidence for the claim that new tubes don't sound as good?

Data or evidence would mean things like tube characteristic curves showing that old tubes are closer to the ideal than new or double blind hearing tests showing that listeners can easily detect the difference.
 
Data and evidence would not include unsubstantiated statements like "in my opinion ABC NOS tubes are way better than XYZ new tubes".

jyprez,

You don't need the data, all you have to do is listen to NOS vs. New.

"new vacuum tubes are not as good as old ones "
That used to be my line of thinking. NOS tubes were made better, better choice of materials, more consistent, etc.

After trying the latest PSVANE Treasure series, not sure I would agree with that original line of thinking anymore.

The sonic qualities on the T-II PSVANEs are so good, I actually prefer them to many NOS types.

this would include input tubes as well as rectifiers. The VANE 5U4G, is an excellent sounding tube,....

Audio components that measure the best do not necessarily sound the best. There are many aspects of music reproduction that we haven’t figured out how to measure yet. Tests that objectively prove what we should like just don’t work in audio, it’s a very subjective pursuit. Can anyone prove that people should prefer Picasso over Van Gogh?

If one prefers current production tubes, that’s what he should buy and enjoy. If others prefer classic tubes that’s what they should buy and enjoy. That way everybody is happy.

So mr_m 
your answer is “no, I have no data or evidence”