Something that has not been pointed out is the performance of the output transformer and the load that is used.
For example the 88db speaker is also 6 ohms. If used on the 8 ohm tap the impedance is transformed by the transformer to a much higher impedance that the power tubes are actually driving. However it will be too low as to perform correctly the 8 ohm tap should be loaded at 8 ohms. The result is that the amp can't make as much power and will have higher distortion.
On top of that the bigger the transformer, the more its going to have troubles making full bandwidth. These problems are compounded on the 4 ohm tap and especially if the tap is incorrectly loaded. You might see as much as an octave of bass response lost on this account!
However we see that transistors are not immune to the issue of impedance. While it is true that many transistor amps can drive lower impedances with more power, it is also true that those same amps will have more distortion while doing so. The ear translates many forms of distortion into tonality; IOW just because a transistor amp can drive the lower impedance is not at all the same as saying it sounds its best.
So here is the bottom line, tube or transistor:
"If sound quality is your goal, it will be best served by a speaker that is 8 ohms or more, all other things being equal." If you want greater **sound pressure** there is a slight argument for 4 ohms if you have a transistor amp. IOW, there is no argument in support of four ohm speakers in high end audio.
For example the 88db speaker is also 6 ohms. If used on the 8 ohm tap the impedance is transformed by the transformer to a much higher impedance that the power tubes are actually driving. However it will be too low as to perform correctly the 8 ohm tap should be loaded at 8 ohms. The result is that the amp can't make as much power and will have higher distortion.
On top of that the bigger the transformer, the more its going to have troubles making full bandwidth. These problems are compounded on the 4 ohm tap and especially if the tap is incorrectly loaded. You might see as much as an octave of bass response lost on this account!
However we see that transistors are not immune to the issue of impedance. While it is true that many transistor amps can drive lower impedances with more power, it is also true that those same amps will have more distortion while doing so. The ear translates many forms of distortion into tonality; IOW just because a transistor amp can drive the lower impedance is not at all the same as saying it sounds its best.
So here is the bottom line, tube or transistor:
"If sound quality is your goal, it will be best served by a speaker that is 8 ohms or more, all other things being equal." If you want greater **sound pressure** there is a slight argument for 4 ohms if you have a transistor amp. IOW, there is no argument in support of four ohm speakers in high end audio.