Does a sub need to be on the floor?

There are a few things to remember about subs.  First, bass is largely a matter of moving air, i.e. pressurization and depressurization of the room. Listen to a car stereo to prove this idea with the car's relatively low air volume.  Second, most speakers cannot more air like a sub can since they usually have to be accurate on frequencies higher than low bass frequencies and large cone excursions will cause high bass and midrange problems.  I saw a review of a JL Audio sub where the reviewer felt its bass to be better than that of a Wilson Audio Maxx 2 Speaker, definitely a plumber of the low frequencies.  Third, a lot of speakers cannot produce the lowest bass or bass dynamics but sound great elsewhere.  Example-my single-driver speakers.  Adding a sub enhanced the one weakness of this design, IMO, bass dynamics.  And yes, it took some work.  A lot of where to place a sub depends on the size and dimensions of your room and where you sit.  As you move it away from room boundaries it will have less room gain but may sound better in other ways, particularly if it excites room modes less.  And a lot of recordings are primarily midbass that a sub will not enhance much unless you move its crossover point up into the midbass region, or at least have a slower low-pass rolloff.