Tang Band sent me their PR13-B Subwoofer Bundle. It consists of a 5 inch subwoofer driver and two passive radiators that combine into a very small subwoofer enclosure with a low of 43Hz.
You can get the bundle directly by contacting Tang Band and get drawings for a recommended enclosure here: http://www.tb-speaker.com/products/su…
The 5 inch driver is also available at Parts Express, but the passive radiators is only available directly from Tang Band: https://bit.ly/2rKLoIy
See the Sound Test video below: