I think what Grainger is trying to say is that the power is the constant at each of the impedance settings. The 4 Ohm tap gives 3.5 Watts into 4 Ohms, the 8 Ohm tap gives 3.5 Watts into 8 Ohms, etc.

When you use a 16 Ohm speaker on the 4 Ohm tap, you get less power out (hence your need for for cranking up the level a bit), but with that decrease in power comes a lower noise floor and a higher damping factor.