How important is it to match tubes in a preamp?

I'm not sure about your question, but designers aften design their circuits using the charastic's of specific tubes. As often as not the same pre-amp will have several different types of tubes serving several different purposes. Its not important so long as you keep the same type in each circuit as designed.

What preamp are you talking about? It is more common that the line stage has two of the same tube (one per channel).

Sugarbrie, Bet the 12AU7 is a cathode follower? Just a guess, obviously.

I have a CAT Ultimate that uses a 6922 for each channel (of the line and phono stages) and a third 6922 as cathode follower in each stage. Does the third tube contribute much to the sound? Can I get away with matched pairs of nos tubes and leave the Sovtek 6922 in the cathode follower position?

In my experience the cathode follower contributes the least to sound quality. Try it and see, you can't do any harm - it may well work and save you money.