Most all amps(except the high-end makes) whether they be economy(pyle, pyramid, etc), or mid-grade(rockford, kicker, mtx, etc), will be rated at unloaded voltage(14.4v) playing a 1000hz tone. Loaded voltage(~12.8v) playing at a useable subwoofer frequency(~50hz) the amp will only be able to output about a quarter to a third of it's rated rms power. I know this sounds difficult, but with the technical aspects, I would say a kicker 500rms at 4ohms would be much more useable. On top of that, your voice coil impedance(ohms) will go up within seconds of playing music, thus making your power from amp decrease. As the coil warms up, resistance(ohms) increases.