Speakers have dips in their impedance plot, some of them severe. These dips can create attenuations at the frequencies they appear at. A tube amplifier's feedback circuit will attempt to compensate for this, but it may degrade overall sound quality. I have found that using the 4-ohm tap will help compensate for impedance dips. This only works if the speakers' nominal impedance is 8 ohms. Power output will increase with a descending seen impedance (unless speakers go below 4 ohms) much like a solid state amplifier. You might have reduced power at higher impedances, but your amp's frequency output should be more linear. How this works out for you depends on many factors- but it's worth a try and it costs nothing to implement.