How come that NOS is better them NS new stock ?

Interesting question. There is little doubt that there are sonic differences between different brands of the same type tubes. Many folks like to experiment with the sound of their equipment by changing the tubes until they find the sound they want to hear - just part of the hobby. There is also a general belief that NOS tubes are better than current production (in many instances, but not all, this is probably a correct assessment). And, then there is a group that feel that NOS is always better than the tubes used by the manufacturer (even if they have never heard the equipment involved). That there are differences is not so much in dispute as is how great is the difference and how much are you willing to pay to obtain it.

For me, a more rational approach when first getting into tubes, is to select equipment that you like the sound of with current production tubes. Once you have absorbed its sound, if you identify something you would like to improve you can research possible NOS candidates (or alternative current production). Sort of like the differences in IC's and cables, etc. It should be seen as a tweak.

In regard to your last question, tube design is fairly well settled - not much new coming along. SS design is still changing. There is considerable difference between the design of early transistors and those used today. Its an evolving technology. With tubes is more a matter of the quality of construction, etc.

I think that NOS tubes are better for several reasons:
1-many more tubes were used 50 years ago than today. This meant that economies of scale came into effect-it was cheaper to use good components if you were going to sell more.
2-Since tubes used to be the only option, everyone concentrated on just one thing, tubes. Many of the good NOS tubes were actually made for the military who had very tight standards for tube performances.

Also, from my experiences, many new tubes can sound quite good. Some are even copied designs from vintage tubes. Everyone seems to be trying to copy old Mullard EL34's. However, nobody seems to be able to copy the life span of these. Mullard and Telefunken tubes seem to outlast everyone's efforts-they must be built better.

This is old post, from Tube Asylum that could to some extend answer your question:
"This is mostly economics, IMHO, the more people who buy tubes
and they more money there is (and more competition there
is), the better the quality control gets and the more
effort is made to make better tubes. And indeed that's exactly
what's happening now.
40 years ago, a lot more tubes were being made for audio
purposes since nearly every audio amplifier from kiddie phonos
to stadium amps used them, and the large companies that made them
(like GE,WE,RCA, and Philips) could bring much more financial & engineering
resources to bear on the problems of tube design & QC than
today's smaller concerns can.

The other problem is that there's so many variables in
tube manufacturing that even if the same companies came
back and tried to make exactly the same tubes, they wouldn't
come out the exactly the same. Maybe close enough to
make most people happy, but you'd still get people saying
they could hear the diff.
I think the best you can hope for at this point is that
the current makers make the best tubes they can, in some cases
the results have been pretty impressive. "

Regards, Jack