Ummm.... this is all baloney. The designer chooses, or should choose, the tube best suited to the job. The 12AX7 is a very high mu (gain) tube that is best suited as all or part of the input voltage amplifier in a phono stage. IMO, one never ever needs that much gain for a linestage. However, a linestage does need to have a very low output impedance and the capacity to deliver current. (The two go hand in hand, actually.) Thus you can say that the 6922 and its congeners are best suited to use in a linestage. However, they can also be used to make a very nice phono input stage, by using a pair in cascode, for one example. The 12AX7 has a very high plate resistance and tolerates only a very small amount of current. The high Rp gives it a high output impedance and very little current is delivered, which makes it a lousy choice for a linestage. If 12AX7 is used in a linestage (as is often the case in commercial products due to its cheapness and availability), then one almost certainly needs an additional tube to be used as a cathode follower, to achieve a useably low output Z. The 12AU7, which has the same electrical parameters as a 6SN7, is frequently used for this purpose. (Worst of all is when a 12AX7 is used as CF, which I have seen in some products.) However, IMO, the 6SN7 sounds way way better than any 12AU7 (I have done the listening tests, FWIW). Among medium mu, high transconductance tubes (like the 6922), there are a few other choices, like the 5687 and its relatives, that I personally like better. Then also there are low mu triodes, like the type 27, etc, that sound great in a linestage. Once the proper tube type is chosen for the job, then sound quality will vary depending upon other things, like the linearity of the tube at operating points determined by plate voltage and current, etc, the nature of the plate resistor, coupling caps, etc. SO, it is folly to talk about how a tube sounds without considering these many other factors.