The underlying principle here is that the speaker (enclosure/frame) must be "mass coupled" to the floor (or, preferrably, to the planet!) so that the only thing that can move is/are the drivers. Spikes are simply the easiest and least destructive solution in most cases. But you could, for instance, bolt your speakers right to the floor;--)
And then of course, there's the issue of floor itself: it mustn't move at all. This is often a problem with wood framed (joisted) floors where the subfloor can deform (bend) slightly as it spans between joists. However, there are some tricks (which are obvious to architects and builders) that will minimize the effect of this common type of floor deformation:
1.) The best situation is if the speakers point (front to back) along the same direction as the floor joists. This is most likely to happen automatically if the speakers are placed on the long wall, since joists are usually oriented to span the short dimension of the room. Why is this (long wall) placement best? Because the subfloor's movement (if any) only allows the speaker to rock from side-to-side in this case. But the drivers only move forward/backward, but the direction of the floor joists in this case, prevents the speakers from reacting (rocking) front-to-back; it they did/could, it would rob the drivers of energy they could/should be transferring to the air! This condition would also reduce the drivers' transient response (accuracy in following the signal) -- a problem that particularly degrades the midrange. Of course, they can STILL (possibly) rock from side to side, but that WON'T MATTER unless your speakers have side-firing woofers ;--) Of course there are other acoustic benefits to 'long-wall' placement.
2.) So if you are lucky, and able to orient your speakers in line with the direction of the floor joists as described above, you'll do even better to 'center' the speaker (left-to-right) over the space BETWEEN two joists, as opposed to it straddling a single joist. This will mostly eliminate side-to-side rocking -- although this is less critcal as I explained above.
Implementing these "tricks", if you can, will result in a huge improvement in transient response; e.g. detail and clarity of vocals, plus cleaner (more slam) in the bass response. And it's free ;--)
3.) Unfortunately for many of us, there are multiple reasons we're unable to place our speakers along the long wall, as in 1.) and 2.), however, it's still possible to realize many of the same benefits (minimizing forward/backward rocking) if you will crawl under the floor and actually locate the two joists (now running side to side across the room) which are nearest the speaker -- the one closest in front of the desired speaker location, and the one running closest behind, and mark their location on the floor above. Then center the speaker(s) front to back BETWEEN those two joists -- again, as opposed to the speaker straddling across just one joist. This "trick" will further minimize forward/backward rocking -- the taller/heavier the speaker, the greater the benefit; although, even with monitors on stands, it still really helps to increase the speakers' effective mass by filling those stands with sand or shot. BTW, it's OK to add toe-in to the speakers, but wait until after you've determined the best location relative to the floor joists.
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