Best Isolation Device for Speakers?

Isolation vs coupling...I won't go there. However, if you want to isolate, nothing is better than suspending the speakers from the ceiling. Chains or wire, and perhaps monofiliament fishing line works. For even better isolation (from the ceiling) you could use bungie cords. Cheap too.

With the boxes floating in space you get acoustic isolation as well as mechanical isolation. IMHO the acoustic part is usually most important.

Theaudiotweak...Enclosures should be solidly constructed of "dead" material, shaped so the panels don't vibrate, (cylinders are good), internally braced, and internally damped. If the top/sides of your enclosure vibrate, even glueing the bottom to a concrete floor wouldn't help.

Of course, the real answer is to get rid of the box entirely: go planar.

Zargon...When isolating, the heavier the better! Ceiling joists are similar to floor joists and can easily handle several hundred pounds. In your case I don't think that the fishing line would hack it, but decorative chain, rope, or wire would be fine.

Zargon...I understand the problem. My speakers don't hang either, although I did try it for a while, pre-wife. But I have heard speakers set up this way in a very large room with cathedral ceiling, and it was superb.

Theaudiotweak...The natural period of swinging object (of any weight) on a chain as short as 40 inches is one second, not an audible frequency. The force exerted due to cone motion (its mass and air resistance) is a few ounces, and won't move a 100 pound speaker system enough to be detected. So the idea that the speakers will be swinging around wildly when they play is only believed by those who have never experimented with this setup.

Spikes, or other means of mechanically coupling the bottom panel of a speaker box to the floor may damp vibration of that bottom panel, but will do nothing for the front, back sides and top. Vibrational energy does not need to be "drained" like a fluid from the box. Damping material in and on the box absorbs the vibration which produces heat. The heat does indeed escape from the box, but you can't hear heat.

Theaudiotweak...Have you measured, or even calculated, the motion. I am not sure that you are correct about that. But so what? Any rocking motion of a 100 pound object will be at a very low frequency. So the tweeter moves around a bit. Well, so does the flute, or whatever other instrument is making the recorded sound. No one requires musicians to clamp their instruments into a rigid mount when they play them. Do you clamp your ears into position when you listen?

Again I point out that vibrational energy is converted to heat by damping material. That's its "exit".

So we agree to disagree. :)