Q: I need help understanding what power cords are best!  Especially when traveling long distances. I have been using crappy power extensions not considering proper power. Thinking about getting 10 gauge extensions. Is that necessary?  Will 12 gauge work fine too?

Things do tend to run hotter when voltage drops. When voltage drops, the demand for current rises. There are standards for cable gauge vs distance, and the further you have to go, the thicker the cable should be. The thicker the cable, the less voltage drop there will be at the end, so your amps will have access to more power and, all other things being equal, the cooler they will run.  

If you were running all (say) 6 of your boxes on one extension cable, that may have been the cause of your issue. Essentially the more copper you have carrying the electricity, the better.  It's not as simple as 12 gauge vs 10 gauge because the current demand and the length have to be considered.

Voltage drop calculation

Ohm's Law is a very basic law for calculating voltage drop:

V_drop = I·R

where:

I: the current through the wire, measured in amperes

R: the resistance of the wires, measured in ohms

The resistance of the wires is often measured and given as length-specific resistance, normally in the unit of ohms per kilometer or ohms per 1000 feet. Also, the wire is round-tripped. Therefore, the formula for a single-phase or direct current circuit becomes:

V_drop = 2·I·R·L

where:

I: the current through the wire

R: the length-specific resistance of the wires

L: the one-way length

But, to shortcut the math, you can look at a chart.

To provide 20A at 100' you'll need 8 gauge wire. That's some heavy cable, so the next best thing is to run multiple cables of a lighter gauge and connect less load to each of them.  With four subs combined could draw peaks of 40A, so for long runs you should run 4 extension cables of the appropriate gauge for the length. The tops don't draw much but they will suffer the loss caused by the consumption of the subwoofers. Each of the tops can share a circuit with a sub for instance.