The best way is a thick concrete slab floor with a nicely sized air-pocket before their ceiling. Short of that, it would help to keep the sub away from any walls that extend down into their level. But essentially, if you crank up the sub, they will hear it. Just using the M80s should help a good amount over using both though. The essential problems are that bass radiates in a more omnidirectional fashion than higher frequencies, and for reference: a 30 Hz. wave has a wavelength of ~40 ft. Most cielings don't really amount to very much of a barrier in the face of that wave. Especially in newer construction when the cieling likely consists of little more than plywood and carpet.
