It really doesn't matter which direction the subwoofer driver fires as long as there is an inch or two of clearance before the nearest surface is encountered. Deep bass wavelengths are very long -- one 20 Hz cycle is over 50 feet long -- so they are not affected by nearest-surface reflections like higher wavelengths are. Also, your neighbors won't be able tell if your sub is side or down-firing. Bass energy is omnidirectional and travels very well through solid surfaces, so if it's loud they'll hear it no matter which way it's firing.
