[RodB]

You should match, as closely as possible, the plate load impedance of the output tube to the primary winding impedance by selecting the right transformer primary tap. Sometimes the secondary also has different taps for different speaker impedance. So if the speaker is 3.2 ohms and the transformer has a 4 ohm tap then use that one. If you don't have access to a transformer chart or schematic then you'll have to use trial and error by sending an audio signal through the amplifier, measure the AC signal at the speaker for one set of connections then reconnect the transformer for a different set, rinse and repeat. Use the set that gives the highest output.