An amp will run a given amount of RMS at a certain ohm load. Most common ohm loads are 1 ohm, 2 ohm, and 4 ohm. The voice coil configuration of a subwoofer will determine what ohm load it can run at. Once you know what a sub can run at, you look at how much power it can handle, rms, at that ohm load, then look for an amp that will push that power at the ohm load needed.