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.
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.