How does a tube amp "see" a REL sub?

Since the REL does not apply a high-pass filter to the signal going to the main speakers, as do some other subs, I would think that the impedance curve of the main speakers would be almost completely unaffected, and that the simple equation would hold.

It doesn't matter what you set the crossover point to : the REL doesn't prevent frequencies below this from going to your main speakers, so if your speakers have an impedance dip at 20Hz, and you set the crossover to 70Hz I would bet you'll still see the impedance dip at 20Hz.

It should be entirely eqivalent to wiring a 10k resistor in parallel with your speaker terminals.

That equasion is accurate for simple resistive loads. But a speaker's impedance varies with frequency.

The impedance of the REL sub is in parallel to the impedance of the speakers, since it is connected to the same speaker terminals as the main speakers. When you add parallel impedances A and B you use the equation :

Imp = 1/(1/A +1/B)

Therefore if we have 10,000Ohms in parallel with 8 Ohms the resultant impedance is 7.9936 Ohms.

In a nutshell the REL's high impedance means that the impedance load that your amp sees will not change from before.