Newer homes are getting spec'd with studs on 24" centers out here.
A stud finder works very well for locating studs and no extra holes required
The sub should do fine sitting on the floor. LFE is very hard to cancel but you can certainly try it out with the floor discs or a piece of MDF cut to fit the area of the feet and thus you can reflect the sound waves out/up vs. having them forced into the floor not that I beleive it really matters much. LFE is like a ripple in a puddle of water and it just emminates outward in all directions, but worth a try.
