That equasion is accurate for simple resistive loads. But a speaker's impedance varies with frequency.
How does a tube amp "see" a REL sub?
If you connected a REL sub to a tube amp using the high-level inputs (placing the sub in parallel with the speakers), and you graphed the impedance of the speaker "system" (speakers + REL) as seen by the amp, would the impedance of the system be 100,000 Ohms up to the crossover point defined on the sub? Could you assume that the REL would be a good sub to mate with tubes because of the highly-resistive (100,000 Ohms) load the sub presents?