Product review (5 stars): Fantastic Subwoofer Isolation Pads
I won't bother writing a review on the benefits of using these to quell vibrating washing machines--others have already covered this.

I recently purchased an Epik Conquest subwoofer--18" driver, 1000W RMS amplifier, and 200 LBS of MDF.  This thing moves air and it was also shaking the house.  I had read that subwoofer isolation was important to sound quality, but I was put off by the cost of audiophile subwoofer isolation pads!

I decided to try these, and I think they definitely make a difference!  Bass sounds much more controlled and less boomy now.  While the subwoofer still shakes the floor, it is from the movement of air, and not the cabinet vibrating the floor.  At less than half the price of other subwoofer isolation kits, I would definitely recommend trying these first!  If they don't meet your needs, put them under your washing machine as designed and get the more expensive subwoofer isolation pads.