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.
Best Speaker Impedence for Tube Amplifiers
I have read conflicting information regarding the best speaker nominal impedence match for tube amplifiers. I know the actual impedence can vary, but all things being equal does a tube amp have an easier time driving a 6 ohm or 8 ohm speaker? For this question, assume speaker sensitivity is about the same.
Eric
Eric