Preamp Tubes...Lesson 101
Typically contemporarily manufactured preamp tubes last 3 times longer than contemporarily manufactured power or output tubes which are good for about a year, unless they are EL84s which are good for about only 6 months, depending how loud and how often one plays their amp.
Many of these new preamp tubes are excellent sounding even if they aren't as reliable as NOS and fortunately, if one shorts out, no damage is done to the amp unless its socket was wired in wrong. And since they don't cost as much as NOS tubes, I recommend using them unless one prefers the tone of a NOS preamp tube and is willing to pay extra to acquire them. Naturally a NOS preamp tube will outlast a contemporary one so one gets what they paid for, sort of.
The problem with buying some NOS tubes is someone comes across a huge surplus of them, tests them and sells off their rejects. After several generations of this testing and rejecting one could wind up paying an exorbitant amount of money for the bottom of the barrel of rejects resulting in getting an inferior sounding NOS tube. But obviously, this is NOT ALWAYS the case.
Since I strive for good tone as well as reliability and not necessarily for high volume when building or converting amps, this aspect is part of the design.