FIRST, we need to address coupling/decoupling.
Both processes have to do with vibrations of the audio device. The difference is whether the vibrations are created by the device, or if they are external to the device.
All the source components and preamps/amps are for practical purposes not generators of vibrations. Sure, there is some very minor vibration of components with moving parts like disc players (including turntables) but the point is to keep other vibration OUT of those components. They are DEcoupled from the rest of the room typically with some kind of "cushion" that doesn't transmit external vibration into the component.
Speakers, on the other hand, serve no purpose unless they're CREATING vibration. Here's where Newton's laws of motion come in handy: For every action, there is an equal and opposite reaction. Every time the motor structure pushes a driver cone in or out...the whole speaker cabinet is being pushed the opposite direction. Things are worse when the driver is mounted high in the cabinet, so there's a long "lever arm" between the vibrating driver and the base of the cabinet. Only inertia and the mass of the cabinet prevent this cabinet vibration when the speaker is sitting on a big, squishy "cushion" like thick carpet. The result is that the speaker cabinet is providing a low-level, frequency-dependent anti-phase signal to that of the driver (exactly what we DON'T want!)
The point of all of this (pun intended) is that if you use spikes to pierce the carpet and pad, so that the speaker is solidly connected to the TRUE floor, the floor absorbs some of the force that would have the speaker cabinet rocking on the carpet. The strength and mass of the floor stabilizes the speaker cabinet--if the speaker is COUPLED to the floor.
Since the real problem is the speaker cabinet vibrating with the motion of the driver; the thicker and "plush-er" the carpet, the worse the sonic degradation. A speaker cabinet wouldn't need spikes if the normal plastic feet were in contact with the bare floor. The purpose of spikes is to pierce the squishy floor covering, and in the case of carpet, since it's a woven fiber, the spikes do no more damage than a sewing needle does to the fabric of your clothes. Hardwood floors probably do NOT need spikes, although the stabilizing effect is probably less than with concrete due to the wood floor's relative lack of strength and mass. For God's sake, do not fall into the marketing crap whereby first you have to buy spikes to couple the speakers to the wood floor, and then you have to buy protective cups so the spikes don't damage that same floor. Either spike to the true floor...or just leave the OEM plastic "feet" in place. Protective cups do the exact opposite of spikes, using them together is a double waste of time, money, effort, and enthusiasm.
On typical "box" speakers, I prefer three spikes to four--that way there's no problem similar to having a four-legged restaurant table, with one leg a touch too short.
I would put a power drill to my head rather than buy >$30 spike sets. I make them by installing tee- or hurricane-nuts into the speaker base (if a threaded insert isn't there already) and then buying nuts and bolts of the appropriate length and thread pitch at any hardware store. Cut the heads off the bolts, sharpen the bolt shank on a bench grinder, and when final-adjusted for stability, tighten the jam-nuts to lock the studs in place. 1/4-20 nuts and bolts are my first choice, but if there are already threaded inserts in the speaker cabinet base, buy whatever size nuts and bolts fit the existing threads. If the spikes are short enough, they'll be invisible and give the appearance that the speaker cabinets are resting on the carpet--so there's no need to buy "pretty" spikes. Home-made spikes would cost less than $5 for a set of six...plus some labor.
