One Dual Voice Coil Speaker in Series:

Dual 8 Ohm Subwoofer: 16 Ohms

Dual 6 Ohm Subwoofer: 12 Ohms

Dual 4 Ohm Subwoofer: 8 Ohms

Dual 2 Ohm Subwoofer: 4 Ohms

Dual 1.5 Ohm Subwoofer: 3 Ohms

One Dual Voice Coil Speaker in Parallel:

Dual 8 Ohm Subwoofer: 4 Ohms

Dual 6 Ohm Subwoofer: 3 Ohms

Dual 4 Ohm Subwoofer: 2 Ohms

Dual 2 Ohm Subwoofer: 1 Ohm

Dual 1.5 Ohm Subwoofer: 0.75 Ohm

Two Dual Voice Coil Speakers in Series / Parallel: