High gain 12AX7 tube? If used in a single tube per channel phono section, you need all the gain you can get.
Low noise? Again in a phono section this is important, especially in the first tube in any circuit.
Matched sections? This is most important if the two triode sections of a tube are sharing two separate audio channels, one triode per channel. Then you definitely want matched gain between the two sections.
Matched sets of tubes? Of course any of the selections offered means that the tubes you get have had more testing than just individual tube selections. This probably lowers the risk of infant mortality. We do this in our Ultravalve tube amplifier, ordering matched quad sets of output tubes and very rarely do we have a premature output tube failure.
Note that there is a difference in gain between sample to sample of any brand and type of tube. There is usually a bigger difference in gain from brand to brand.
In general, too much tube gain for a given circuit increases feedback and feedback related transient distortion, causing excess brightness and listen fatigue.
Too low a tube gain for a given circuit reduces feedback, possibly too much, causing increased harmonic distortion and muddy sound in extreme cases.
It is nice to know what the designer of the circuit really wanted to select the best tubes for that circuit. In randomly doing "tube rolling" you results may vary substantially, and not because of the apparent brand, age, or other advertised virtues, just because of gain differences.
A tube tester is a useful device, kind of like having a tire pressure gauge when making tire tests.
Frank
