The large majority of 12AX7/ECC83 tubes are classified by degrees, from A to D.
A is a tube whose values are near or slightly above the manufacturer's datasheet, so Ia:1.2 mA, Ug:-2, for Ua:250V and D is a tube in a perfect operating but rather in the 50% (between 0.6 and 0.7 mA).
The problem is that a pre-amp tube is considered bad below 0,8 mA.
That's why there are pre-amp tubes whose price varies between 7 and 10 €. These tubes come directly from factory without really being controlled. These are rather in values 0.7 / 0.8 mA, with a very dubious internal construction (in all brands!). The tubes that are between 15 and 20 € (or more) were pre-sorted and renamed under the larger amp manufacturers brands (MesaBoogie, Marshall, Fender ...).
Unfortunately, sometimes the brands mentioned above may also sell faulty tubes.
Other factories in England, USA or Japan sort their tubes and rename, but all this has a cost that raises the invoice between 20 and 30 € for standard brand tubes.
You should also know that in the 50s, 90% of the tubes were of good quality, they come from the USA, Russia or England ... But this percentage, nowadays, fell unless 40% . The reason is simple, only the music world today still use some tubes references.
When the tubes are in manufacturer's good datasheet values, their life span is decreasing continuously, due to the number of hours of use.
The life of the tubes will also depend on bias adjustement and voltage reading on each tube. A bias-meter is then indispensable.
Of course, the tubes must be matched, especially for power tubes.
In my workstation, I invested in a professional tube-tester, the handwired FC2013 by Christian Fabert, which allows me to apply several tests on all commercially available tubes (vacuum test, transconductance measurement, internal impedance value, amplification coefficient mesurement, isolated test ...).
My objective is to avoid replacing all tubes of the amp and raise the customer bill unnecessarily.
(Clic to enlarge picts)