Here is the rule of thumb:
"all other things being equal, your tube amplifier performance dollar is best realized by the use of a higher impedance speaker- 8 being better than 4, and 16 being better than 8".
Tube amps in general will make lower distortion and have wider bandwidth, without loss of power into higher impedances. This has to do with the efficiency of the output transformer (if the tube amp has no output transformer this rule is still true). So 8 ohms is preferred even over 6 ohms.
Its funny but this rule applies to transistor amps too, although for very different reasons (hint: there is a difference between making more power and making the best sound).
Now the other thing you mentioned was the speaker Sensitivity being the same. Sensitivity is measuring the output of the speaker based on a given input voltage, which is 2.83volts. **Efficiency** on the other hand, is measuring the output of the speaker based on 1 watt input.
At 8 ohms the two are the same. At 6 ohms they are not- in fact the speaker that has the same sensitivity at 8 ohms is a more efficient speaker.
If this is confusing, take a look at this link:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
The article at the link will give you a greater context.
"all other things being equal, your tube amplifier performance dollar is best realized by the use of a higher impedance speaker- 8 being better than 4, and 16 being better than 8".
Tube amps in general will make lower distortion and have wider bandwidth, without loss of power into higher impedances. This has to do with the efficiency of the output transformer (if the tube amp has no output transformer this rule is still true). So 8 ohms is preferred even over 6 ohms.
Its funny but this rule applies to transistor amps too, although for very different reasons (hint: there is a difference between making more power and making the best sound).
Now the other thing you mentioned was the speaker Sensitivity being the same. Sensitivity is measuring the output of the speaker based on a given input voltage, which is 2.83volts. **Efficiency** on the other hand, is measuring the output of the speaker based on 1 watt input.
At 8 ohms the two are the same. At 6 ohms they are not- in fact the speaker that has the same sensitivity at 8 ohms is a more efficient speaker.
If this is confusing, take a look at this link:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
The article at the link will give you a greater context.