Speaker spikes OR Rubber feet ?

Go with the spikes. Never could figure whether spikes "coupled" or "decoupled" the speaker from the cabinet but every speaker I've added spikes to sounded so much clearer, more detailed and transparent whereas rubber feet don't do much at all. You will not be sorry going with spikes. You can purchase some do-it-yourself spike kits on line from Parts Express - they carry good, yet inexpensive spike kits from Dayton Audio which even include metal disks for under the spikes to protect your wood shelves. I used the 1" spikes under my VonShcweikert VR1s and they made a great speaker truly outstanding.

Good luck.

I put 1" squares of styrofoam under a pair of speakers with positive results.

What about 4 Vibrapod Isolators under each speaker? My KEF Reference Twos are on a tile ledge and I've been wondering if adding 4 Isolators under each speaker will improve the sound?

My experience matches that of Jgiacolo. Always an improvement. As he points out, the Dayton kits with the discs are a good deal.

Rubber feet will work best on the top shelf of a piece of furniture. Rubber
feet will also preclude any discussions about scratching the wood. I have
Omega Super 3 monitors (15 X 8 X 10, h/w/d) sitting on their sides (they
become 8 X 15 X 10). I have used rubber erasers in the past ... the 4 for $2
variety that you can pick up in Staples (the side physical placement works to
my advantage here).

Lately, I have been using dark brown hard rubber furniture leg cups ... the 4
for $3 variety that you can pick up in Home Depot. I use three underneath
each speaker. The rubber does a good job of dampening the vibrations (a
bus route goes down my street) and contribute to overall sound ... but don't
expect a night and day difference ... few things are.

If your speakers have some weight to them, you can also use rubber hockey
pucks (stay away from the ones made out of pvc, like the souvenir pucks).
That would be another $1 per rubber block solution.

You should not have to pay a small fortune to dampen vibrations. So many of
the products out there have way out of line pricing for what they deliver.

Regards, Rich