I am somewhat with stevecham on this one, not that I can't tell the difference in sound, but it is not a sure thing that NOS will always outperform current production tubes.
I believe, like many things in audio, it is just a matter of matching. Different brands of tubes (NOS or current production) do have different sound, so the users have to match them to their systems. For example, in one of my amps, I prefer the plain EH6922 to NOS Amperex 7308.
Think about it, most current tube amp manufacturers are probably tuning the sound of their amps with current production tubes, so NOS tubes probably don’t have any advantages.