On one side of the transformer (the primary), you have the output section of the amplifier, and on the secondary coils, you have the speakers. Transformers work on a ratio system, so what happens on one side has to happen on the other- or else you end up with an imbalance, and mismatched impedance. By removing two of the valves from the power section, you halve the impedance, so you need to half the impedance on the speaker-side of things.