Transformer
e.g. two 8 Ω cabs in parallel = 4 Ω
How it works
An output transformer does not have "an impedance" of its own: it has a turns ratio, and it transforms whatever load is connected to the secondary by the square of that ratio. The "6.6 kΩ : 8 Ω" printed on a transformer simply means: with 8 Ω connected to the 8 Ω tap, the valves see 6.6 kΩ.
Connect a 4 Ω cabinet to the 8 Ω tap and the primary impedance halves; connect 16 Ω and it doubles. Valve amps are fairly tolerant of a one-step mismatch; many players like the different feel, but the valve must never run with no load: the reflected impedance becomes infinite and inductive voltage spikes can arc the socket or the transformer.
Push-pull per-valve load
The primary of a push-pull transformer is centre-tapped. Impedance scales with turns², so half the winding presents a quarter of the full impedance. In the class B region only one valve drives its half and sees Raa/4. In the class A region both valves work together, each effectively seeing Raa/2. Class AB load lines are therefore drawn with Raa/4, bending to Raa/2 around the idle point.