it depends on the amp - in particular the output transformer. The one rule I've seen is when your amp has multiple output taps (eg 4 ohms, 8 ohms, and 16 ohms), you are best off using the one that uses the greatest amount of windings - which is the highest ohm rating (16 ohms in my example). Of course this assumes your speaker impedance must match the tap rating. You wouldn't want to put an 8 ohm speaker on a 16 ohm tap - it may work, but it also may ruin your tubes or more. If you have a 6 ohm speaker and 4 ohm and 8 ohm taps, you can try both and see which sounds best.
4 responses Add your response
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:
The article at the link will give you a greater context.
It's really trial and error. What sounds better to you. For example, in some older Audio Research Amps, the 4 ohm tap can actually sound better with a 6 ohm dynamic speaker. With dynamic speakers it's all about current delivery controlling the woofer making the 4 ohm tap more useful. With an electrostatic speaker, it's all about voltage swing so the 8 ohm tap might be more useful. Here current delivery isn't important unless the speaker uses a dynamic woofer that is not powered, in which case bi-amping might be advantageous. Tubes for the electrostatic and solid state for the dynamic woofer.