The best way to isolate my subs from the floor?

Try something with bicycle innertubes. I used these wrapped around some home-made Sonotube-based subs, with good effect. I have 16" subs, put 3 inner tubes around it, lightly inflated them - worked quite well. Try a platform of MDF with sub on top and innertube flat underneath it.

-Ed

I have seen Vibrapods recommended several times for use under speakers. They are about $6.00 each and are available from several sources. They would be the least expensive option posted so far, won't harm anything, can be easily removed if you don't like the sound, and can also be used under other components.

I have three 1" superballs that are cut in half. Three of the halves are on the floor supporting a piece of MDF. The other three have are between the MDF and the sub. This "double isolation" works great.

Be very careful as you may get what you ask for. Until a year ago I used two pairs of Entec lf40's which had very extended low frequency response. After hearing a friend's system with the same speakers who had put spikes underneathe them, I tried doing the same on my very solid wood floor. This was an unmitigated disaster. The bass seemed to disappear. In speaking with my friend, I learned that his floor was carpeting over a concrete slab.

After some reading, I discovered that wood floors are very similar to drum heads with a very complex pattern of resonances which varies depending on the particular place on the floor that you site something. Concrete slabs apparently flex less and are more predictable.

The moral is not to try anything which you cannot return if it does not work.

I use 3 inch dia solid aluminum legs. I then put 1" spike under each 6 inches tall cylinder. 1/2 inch Rubber feet between the speakers and each Al legs. Depends on what's your dimension. I used total of 60% surface area of bottom of the speaker as the total size I need for the alumium.

You can go crazy with what works for you....
You just need to know how much control and isolation you need.