$700 used for an Empire is almost what it cost new. Not a good buy. Drivers will likely last a long time, but the amps are an unknown and I've already had mine replaced once...and will obviously have to make it passive if it goes again. If $700 is your range, why not a PSA, SVS, HSU or Rythmik? PLENTY of solid choices.
I wouldn't hesitate to recommend a SVS over Earthquake.