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.
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