Rythmic Audio used to offer a down-firing sub. Designer Peter Ding found it to be inferior to all his front-firing models, and discontinued it.
In talking about the wavelength of bass frequencies, remember: the lowpass x/o filters are not brickwall. If you are using a 1st order filter at 80Hz, the sub is down only 3dB at that frequency, and 6dB at 160 Hz. That is clearly audible, and will effect one's perception of the sub. The steeper the filter and lower the x/o frequency, the less will you hear the sub above the low bass.