Down firing will reduce the audibility of harmonic distortion. It generally makes placement easier and will increase the sub efficiency (like a horn - the coupling to the air will be improved at a room boudary). It also means that a huge cone and large voice coil and suspension will not have gravity acting across it from one side...think of a yo-yo - it will remain most balanced and controlled going up and down and is relatively harder to control going straight out in front of you - as gravity has a tendency to make pull it out of alignment and you have to fight this pull to keep the yo-yo going out straight in front of you.
Some people recommend removing front firing subwoofers and rotating them by one screw point each year to stop any gravitational affect eventualy causing cone misalignment.
Some people recommend removing front firing subwoofers and rotating them by one screw point each year to stop any gravitational affect eventualy causing cone misalignment.