Like its front-firing cousin, the SB-900DF offers an 8" woofer powered by a 125 watt amplifier.  It goes down to a frequency of 32Hz and up to 200Hz at a peak output of 102dB SPL.  It sports distortion-detection circuitry that helps avoid unpleasant sound and takes advantage of "floor coupling" which improves the sub's bass response.  Like the SB-900, the SB-900DF is available now at a price of $349.