Bestbuy offers MartinLogan - Dynamo 400 8" 150W Ported, Compact, Powered Subwoofer - Satin Black for $299.99 (orig. $499.99) with free shipping.

Product Description:
Upgrade your sound system with this MartinLogan Dynamo 400 subwoofer. The high-excursion 8-inch woofer and powerful amplifier provide up to 150W of clear, booming audio with a frequency response between 30Hz and 200Hz. This MartinLogan Dynamo 400 subwoofer supports both two-channel and multichannel setups thanks to its versatile input and output ports. 

• Integrated Class D Amplifier
  Dynamo 400’s Class D amplifier supplies 150W peak power (75W RMS) and operates with extremely low levels of distortion to deliver dynamic impact with plenty of overhead.
• Low Pass Filter and Phase Adjustment
  Properly calibrate the subwoofer for seamless integration into any system.
• Inputs
  Dynamo 400 features Stereo RCA Inputs, a RCA LFE Input and speaker level inputs for maximum flexibility.
• Low-Turbulence Port
  Dynamo 400 features tuned, long low-turbulence ports designed to provide superb low-end extension while minimizing port induced distortion.
• Inverse Mathematical Equation (IME) Filter
  The IME filter used in the Dynamo subwoofers is derived from the inverse of the unfiltered driver response. Precision IME filtering yields extremely accurate low-frequency response.