Pick your driver(s), how each one's voice coil(s) are wired, and how multiple drivers are combined — get the final impedance your amplifier actually sees, plus a diagram of the exact wiring you picked. Uses this site's own driver database's coil-configuration data, so a DVC sub's real per-coil rating drives the math, not a guess.
Everything above is the driver's DC/nominal impedance — the number on the box, and the number your amp is actually wired for. But a real voice coil's electrical impedance isn't flat with frequency: it rises above that nominal number near the driver's own resonance, and again near a ported box's tuning frequency — a normal, well-documented electrical property of voice coils and enclosures, not a wiring mistake or a bad amp. See the Troubleshooting page's "amp seems to run out of power near tuning" row for the symptom-side explanation of the same effect. Wire to the final impedance calculated above; don't chase the resonance bump as if it were a mis-wire.