If the amplifier has an output impedance that is low enough, you can have variations in impedance that will not result in tonality issues with most tube amps, as long as the variations are not extreme. For example, a 4 ohm dip at a crossover frequency can be considered benign.
A lot has to do with the intention of the designer of the speaker- if the speaker is designed to be friendly to tubes, that is usually more important than the actual impedances.
A lot has to do with the intention of the designer of the speaker- if the speaker is designed to be friendly to tubes, that is usually more important than the actual impedances.