I'm not here to argue about the sound differences because some of us hear differences between tubes and some of us don't. There are so many small things to be taken into consideration when doing critical listening that I'm not even going to start. I just want to note that during the golden years of tube technology there were no restrictions of what materials could be used in tube production. Therefore, tubes of today do not necessarily have the same materials as what the NOS tubes had. Some rare metals, for example, were then more readily available and weren't as expensive as today. NOS tubes were also designed for high end purposes (military/technology/medical uses) and lots of love and dedication went into the tube manufacturing (from refining to assembling).
The reality of NOS.
Steel was really coming into it’s own around World War 1. Submarines needed metals that could be compressed at depth over and over. Airplanes need crankshafts that could stand up to their job. Metallurgy continued to improve. There were great steel companies such as US Steel in the USA, and the fine Swedish and Finnish steels. Today, most of the US steel companies are gone. The great controlled steel recipes are reserved for more critical applications such as aerospace or the automotive and aircraft industries, where a lawsuit is just around the corner. If a tube fails in an amp, who cares?
Then, there is economics from a geographic point of view. The Russians are not cranking out defense equipment any more. They have no money. One batch of steel is completely different than the next batch. In Eastern Europe, it is the same when it comes to the quality control of metal alloys. There is little economic reason to have high quality control standards in a cheap vacuum tube.
When tubes were used in scientific, medical, and defense equipment, quality was an issue. Today it is not an issue.
So, where does this get us?
Today’s new tubes are very inconsistent. Their specs run plus to minus 50%. They are not linear (those plate alloys react differently as the frequency changes and heat changes for one thing), and they do not meet specification that were established in 1957. They may meet one spec, but only if you are lucky.
Even though there are lots of tube measurements, electronics and frequency graphs available, I only trust my own ears.