Wiring for a possible future rear subwoofer:

If the future subwoofer is going to have a built-in amplifier, then you will need a good quality shielded cable run to the rear. If you don’t know whether that sub will have a balanced or unbalanced connection, I would have a cable run for balanced (two insulated conductors within a shield) signals. If you later run unbalanced, the balanced cable can easily be wired as unbalanced.

If the amp for the subwoofer is going to be separate and only the driver/enclosure will be at the rear, then the larger the gauge, the better. Woofers with large excursions do much better with good damping, wire of too small a gauge will do away with much of the damping factor. If the cable is being run over a distance of fifty feet or so, that means the signal will travel through about 100 feet of wire for a round trip. In such a situation my choice would be about 12 gauge copper wire. If the run is 75 feet or more, I would look to install 10 gauge. Others have installed lighter gauge wiring with no problem, but I don’t know what impedance the sub’s drivers will be. My personal preference: for lower impedance drivers and longer the cable runs, the lower the resistance of the cable should be.

See these charts for reference:

http://www.powerstream.com/Wire_Size.htm

http://www.epanorama.net/documents/wiring/wire_resistance.html

Note that they don’t agree on the exact numbers, but they do agree in principle.