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.

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.