Should you buy NOS tubes or new tubes when purchasing tubes or sourcing replacement tubes?

First, you have to check whether the tube type you are looking for is available as new production. For most HiFi tubes, for example EL34 or KT88, high-end audio devices, for example 300B or 845, as well as guitar and bass amps, for example 12AX7 or 6L6GC, tubes from current production, such as JJ or Tung-Sol, can be purchased. Typical radio tubes, for example ECH81 or EF85, are usually only available as NOS stock, so the question does not really arise there. Newly produced tubes are cheaper and can be matched or selected much better, because large quantities can be measured. With NOS tubes, availability is limited and demand is high, so the price is significantly higher. Sonically, NOS tubes are not automatically better. It is best to buy selected quality tubes, for example S4A or S4GB tubes, or high-quality branded tubes, for example Genalex or Linlai, plus selection where available.