If you have an 8 ohm speaker, use the 8 ohm tap. If you use the 4 ohm tap, the transformer will not be loaded correctly and will 'ring' that is to say it will add distortion, which usually makes the amp sound brighter (though not always).
Conversly, if you put a 4 ohm speaker on the 8 ohm tap you just cut the load that the tubes see. For example if the tubes are seeing 3000 ohms 'plate to plate' the impedance just dropped to 1500 ohms. This will cut power (maybe by as much as 1/2) and add distortion! Transformers are called that because they transform impedance and that goes both ways.
Conversly, if you put a 4 ohm speaker on the 8 ohm tap you just cut the load that the tubes see. For example if the tubes are seeing 3000 ohms 'plate to plate' the impedance just dropped to 1500 ohms. This will cut power (maybe by as much as 1/2) and add distortion! Transformers are called that because they transform impedance and that goes both ways.