Amazon Warehouse Deals offers the used Polk Audio 50-watt 10" Powered Subwoofer in Black, model no. PSW10, for $63.47 with free shipping. That's $17 under our mention of a new unit from November and the lowest total price we've seen. (It's also $59 below the best price we could find for a new unit today.) It features a medium-density fiberboard enclosure, frequency response of 35Hz to 200Hz, and line-level and speaker-level inputs.

Note: No warranty information is provided.