To DCStep
Many tube equipment when sold also do not provide you the best tubes, the reason being that it's quite hard to find the best tubes which are NOS, new old stock, which are no longer in production. They are limited in numbers, and the prices rise each year for many of these good tubes. And no tube audio equipment manufacturer will cost in their equipment or otherwise their equipment will fluctuate monthly with the price of these NOS tubes.
I guess you never used any tube audio equipment before. ;-)
As to why NOS tubes are still the best, google and find out yourself
Many tube equipment when sold also do not provide you the best tubes, the reason being that it's quite hard to find the best tubes which are NOS, new old stock, which are no longer in production. They are limited in numbers, and the prices rise each year for many of these good tubes. And no tube audio equipment manufacturer will cost in their equipment or otherwise their equipment will fluctuate monthly with the price of these NOS tubes.
I guess you never used any tube audio equipment before. ;-)
As to why NOS tubes are still the best, google and find out yourself