i would say that servo control is a factor that makes a good sub. from my manual:
"The accelerometer (closed-loop, i put this in since it just said it above the quote) continuously monitors cone motions, providing us with a feedback signal for the servo control circuitry. This high-gain system is designed to improve linearity and reduce distortion approximately 30 times. over conventional non-servo systems. While the input signal is being sent to the amplifier which powers the driver, a continuous signal representing cone motion is being sent to a comparator circuit. This circuit constantly adjusts for any deviation between the input signal and the subwoofer's output. This self regulation results in extreme control over cone movement, and very accurate low distortion sound."

Ironically below this paragraph they talk about the amplifier as extremely efficient Class D, and cool at 1250rms. that contradicts what sh said about them being so inefficient.