Pubul57, the act of increasing the bias on the tubes has the effect of reducing their output impedance. This will change the relationship they have to the output transformer.
In addition, you have to also consider how the amplifier was designed. For example, many power tubes want to see about 3000 ohms plate to plate. But what if you had the transformer designed to be 3500 ohms plate to plate? You might loose a little power, but now you can experiment with different taps to affect the sound in different ways. This is because the transformer really does what it is called- it transforms impedance. The load that is on the output taps will affect the load that is on the tubes. Especially in the last 30 years, its been a good idea to build in a little reserve to deal with the many 4 ohm speakers out there.
So if in the above case you are loading the 4 ohm tap with an 8 ohm speaker, the result is that the tubes might see a load impedance that is much higher, perhaps 6000 ohms. Now in some cases the amplifier will not be able to make as much power, as the voltage that needs to be made across such a load to get the power is no longer available. But it may not matter if the power thus obtained is sufficient.
