I agree with everyone else, the ceiling is probably not a good place for a sub. It may sound ok until all of your lights rattle loose and the plaster cracks from vibrations.
They do make subs for under floor installation. All you need for this is a small vent in the floor (ie an air conditioner register) for the sub to work. These subs are designed to fit between standard 16" floor joists quite nicely. I saw them not long ago at either mcmelectronics or parts express. You could always use a niles, snell or sunfire in wall sub under the floor also. JUST REMEBER: once it is installed, you can't move it. It may be worth having a pro do this with proper equalization?
They do make subs for under floor installation. All you need for this is a small vent in the floor (ie an air conditioner register) for the sub to work. These subs are designed to fit between standard 16" floor joists quite nicely. I saw them not long ago at either mcmelectronics or parts express. You could always use a niles, snell or sunfire in wall sub under the floor also. JUST REMEBER: once it is installed, you can't move it. It may be worth having a pro do this with proper equalization?