Brent Butterworth measured the SUB-1500 and the SUB-1000L (
@j_garcia's flat version), as well as a bunch of other <$250 subs. See
this interactive graph. You'll see that at 20Hz, the SUB-1500 (as well as the BIC V1220) is about 6dB louder than the SUB-1000L.
But a budget of ~$500 opens up better alternatives -- a few that
@shadyJ mentioned, as well as the Rythmik LV12R, the RSL Speedwoofer 10S, and perhaps others.