The following quoted claims are taken straight from the websites of manufacturers of these devices.
The Isolation Claim: ‘Its purpose is to prevent sound from transmitting through your subwoofer to surrounding surfaces. Subwoofers create big vibrations (low frequencies) that you can feel in the floor and in objects placed nearby. When the source of the vibrations is coupled directly to the floor it causes these objects to vibrate or resonate…’
The Truth: The source of these vibrations is the movement of the driver cone creating pressure waves in the room. The claim would only be true if you coupled the driver cone to the floor. If the cabinet panels vibrate enough to cause the floor to vibrate the speaker is defective.
The Decoupling Claim: ‘Isolators for your speakers…will decouple your speakers from the surface they rest upon, resulting in a more pure, accurate tone. Low frequencies will be projected and will no longer lack the definition you desire. Mid and high frequencies will be crisp and intelligible. Rattles and resonances will be a thing of the past.’
The Spike Claim: ‘By rigidly coupling a loudspeaker enclosure to a floor by means of a spiking system, it is possible to dramatically improve clarity, stereo imaging and bass response. This is very apparent with subwoofer systems.’
The Quandary: These sources claim the same benefits from coupling and from decoupling. Who’s telling the truth?
The Truth: Both are lying. Isolation and coupling makes no difference. To test this I measured the response of my THT and my David with the test mic in the room, in the next room, and in the room below, with the cabinet sitting on the carpeted floor, on four inches of high density acoustic foam, on rubber feet and on spikes. I’d post the measured results for each set of comparisons, but there would be no point. In each case the measured responses of the four options were identical.
Note that this was on a carpeted floor. There may be some slight benefits to isolation devices or rubber feet on a bare floor, or on a bare shelf or stand. But you never want a bare floor, it’s an acoustical nightmare. If you only have area rugs in your listening room stick a piece of felt carpet padding, a carpet scrap or rubber feet under your speaker. If you're using bookshelves on a bare shelf or stand small rubber feet or felt pads are all you need to prevent spurious vibrations.
The Endorser Claim: ‘I tried them and they work, I know what I’m hearing!’
The Truth: The first thing you learn in an acoustical engineering course is that you don’t know what you’re hearing. If you did you’d be able to listen to a speaker, take pencil and graph paper in hand, and draw a frequency response chart, THD chart and waterfall plot, all with 1/24 octave resolution and 1/10dB accuracy. Our ears just aren’t that good, not by a very wide margin. But our imagination works very well, and that clouds our audio judgment, leading to placebo effect. In short, if you think something will make a difference in the sound, it will.
One item that's always missing from both manufacturers and owners claims are measured results, before and after. We often say 'If you don't have a picture it didn't happen'. In this case that picture is measured in-room response. When virtually anyone who's really serious about their sound has REW or the like that's not a lot to ask for.
For an in depth examination of why we really don’t know what we’re hearing check out this video: