EAR 834P tubes: which to match?

I'm upgrading the three Sovtek 12AX7 tubes presently in my EAR 834P phono preamp, and have no circuit diagram (not that it would help me much if I did), and don't know whether all three should ideally be matched, or whether just two of the three should be. My guess is that the outside two should match and that the one in between them need not match them--but that's really just a wild guess.

Does anyone know the answer to this?

I've just finished putting the 834P and a pair of Shallco-Holco ladder attenuators in a wooden chassis, with wires going from the 834P's output jacks to "In" on the Shallco switches, and from "Out" on the switches to output connectors, from which I go straight into my monoblock power amps, cutting out a separate preamp (active or passive) and the cables and connectors it would entail. I can hear a clear improvement in quietness and transparency.
This has nothing to do with my question, but I can't help telling everybody about it these days--sorry!