It's impedance. Volts over resitance. So the higher the ohm load, the more the resistance, the lower the ohm load the less resistance, Im pretty sure. Anyways you buy a woofer that has voice coils (Where you attach your positive and negative wire to). Depending on the woofer it may have 1 voice coil, dual voice coils, or four voice coils. When you buy a woofer it will say dual 2 ohm, dual 4 ohm, etc. This means wiring both coils to your amp will run whatever ohm your woofer is. So lets say you buy a 1 ohm woofer, but your amp doesnt support 1 ohm as he stated above, then you would wire you woofer in series if its a dual voice coil. Wire one of the negative wires to one of the positive wires on the two voice coils, then you have one positive and one negative left (Since there is two voice coils, and each of them have a positive terminal and a negative terminal), then you wire the remaining positive and negative to the amp. Annnyyyways, im just typing to much. If you use that amp (I wouldnt) worry about your wiring for your amp first. Since its a low watt amp you can get away with running low quality wires, so you would only need 4 gauge positive from your battery to your amp, and 4 gauge negative from your amp to your ground. I wouldnt run any higher than 14 gauge to your woofer from your amp and no bigger than 8 gauge. Next find you a woofer, any of the ones you have in mind will work, since your using a meh amp, you can use a meh woofer. I highly doubt your looking at any high end woofers so dont worry about it. And if your not looking for either of those, you probably dont care about your box either. So long story short, any dual 2 ohm, or dual 4 ohm woofer should work fine.