What gauge wire do I need for my subs? How exactly does the volume knob work?

I have 2 subwoofers, 400W RMS, 800W max each, wired at 2 ohms each. I've looked online on http://www.powerstream.com/wire_size.htm... and it doesn't really say much about power, only about amperes, and I have no idea what voltage is on the amplifier output to calculate how many watts or amperes there are. So, the question is - what gauge wires do I need if I decide to wire both subwoofers to my amp in mono, and need to run at least 6 feet, preferably 15 feet long wire? I have 18 gauge wire and plan to use about 15 feet of it to connect the subwoofer - is the wire going to fry when I run 1000 watts through it? Another question - how exactly does the volume knob works? "Turn it clockwise and sound increases" is not good enough. When I turn the knob - does the voltage in the wire increase or amperage? Maybe, both?

P.S. Please stick to the topic and talk about wires and volume knobs only.

Thank you, everybody, for your effort.

When it comes to wire size it doesn't matter what the subs are rated at, it's what the amp can deliver and the distance. That's not to say that the wattage of the subs doesn't matter, it does relative to the amplifier. So for short runs like yours, if your amps power rating is close to your speakers power handling, then 14 would be OK, I would error to larger like 12 or 10 if you are really going to crank it. 18 is far too small for the kind of power you are talking about.