'NOS' tubes are one of the bigger problems we have with our preamps. It is very difficult to find NOS 12AT7s that are as quiet as actual new ones. As with any high-performance tube equipment, you can't just buy tubes off-the-shelf and have them work 100%. Usually you have to hand-pick them.
So in practice, when people have bought NOS 12AT7s for use in our preamps' phono sections, they are often buying tubes that are significantly noisier than what we regard as acceptable- then they assume that the preamp is noisy! Before we even audition a tube, we start by grading it in a tester. It must read about 80% higher than the 'passing' value to even be considered for use. Incidentally, that number is typical of 'testing like new' for 12AT7s, anyway...
So in practice, when people have bought NOS 12AT7s for use in our preamps' phono sections, they are often buying tubes that are significantly noisier than what we regard as acceptable- then they assume that the preamp is noisy! Before we even audition a tube, we start by grading it in a tester. It must read about 80% higher than the 'passing' value to even be considered for use. Incidentally, that number is typical of 'testing like new' for 12AT7s, anyway...